Add latest changes from gitlab-org/gitlab@14-3-stable-eev14.3.0-rc42

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diff --git a/doc/integration/mattermost/gitlab-mattermost.msc b/doc/integration/mattermost/gitlab-mattermost.msc
new file mode 100644
index 00000000000..f6d4bf7aa68
--- /dev/null
+++ b/doc/integration/mattermost/gitlab-mattermost.msc
@@ -0,0 +1,28 @@
+msc {
+  # Use https://mscgen.js.org or mscgen to convert this into PNG
+  hscale="1.5",
+  wordwraparcs=on;
+
+  user [ label="User", textbgcolor="blue", textcolor="white" ],
+  mattermost [ label="Mattermost", textbgcolor="red", textcolor="white"],
+  gitlab [ label="GitLab", textbgcolor="indigo", textcolor="white"];
+
+  user=>mattermost [label="GET https://mm.domain.com"];
+  mattermost note gitlab [label="Obtain access code", textcolor="green"];
+  mattermost=>gitlab [label="GET https://gitlab.domain.com/oauth/authorize", textcolor="indigo"];
+  gitlab rbox user [label="GitLab user logs in (if necessary)"];
+  gitlab rbox gitlab [label="GitLab verifies client_id matches an OAuth application"];
+  gitlab=>user [label="GitLab asks user to authorize Mattermost OAuth app"];
+  user=>gitlab [label="User clicks 'Allow'"];
+  gitlab rbox gitlab [label="GitLab verifies redirect_uri matches list of valid URLs"];
+  gitlab=>user [label="302 Redirect: https://mm.domain.com/signup/gitlab/complete"];
+  user=>mattermost [label="GET https://mm.domain.com/signup/gitlab/complete", textcolor="red"];
+  mattermost note gitlab [label="Exchange access code for access token", textcolor="green"];
+  mattermost=>gitlab [label="POST http://gitlab.domain.com/oauth/token", textcolor="indigo"];
+  gitlab=>gitlab [label="Doorkeeper::TokensController#create"];
+  gitlab=>mattermost [label="Access token", textcolor="red"];
+  mattermost note gitlab [label="Mattermost looks up GitLab user", textcolor="green"];
+  mattermost=>gitlab [label="GET https://gitlab.domain.com/api/v4/user", textcolor="indigo"];
+  gitlab=>mattermost [label="User details", textcolor="red"];
+  mattermost=>user [label="Mattermost/GitLab user ready"];
+}
diff --git a/doc/integration/mattermost/img/gitlab-mattermost.png b/doc/integration/mattermost/img/gitlab-mattermost.png
new file mode 100644
index 00000000000..1eedcb391b0
--- /dev/null
+++ b/doc/integration/mattermost/img/gitlab-mattermost.png
diff --git a/doc/integration/mattermost/index.md b/doc/integration/mattermost/index.md
new file mode 100644
index 00000000000..4830f8ba84e
--- /dev/null
+++ b/doc/integration/mattermost/index.md
@@ -0,0 +1,495 @@
+---
+stage: Enablement
+group: Distribution
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
+# GitLab Mattermost
+
+NOTE:
+This document applies to GitLab 11.0 and later.
+
+You can run a [GitLab Mattermost](https://gitlab.com/gitlab-org/gitlab-mattermost)
+service on your GitLab server. Mattermost is not part of the single application that GitLab is. There is a good integration between with GitLab, and our Omnibus installer allows you to easily install it. But it is a separate application from a separate company.
+
+## Prerequisites
+
+Each release of GitLab Mattermost is compiled and manually tested on an AMD 64 chipset for Linux. ARM chipsets and operating systems, like Raspberry Pi, are not supported.
+
+## Getting started
+
+GitLab Mattermost expects to run on its own virtual host. In your DNS settings you will need
+two entries pointing to the same machine, e.g., `gitlab.example.com` and
+`mattermost.example.com`.
+
+GitLab Mattermost is disabled by default. To enable it:
+
+1. Edit `/etc/gitlab/gitlab.rb` and add the Mattermost external URL:
+
+   ```ruby
+   mattermost_external_url 'https://mattermost.example.com'
+   ```
+
+1. Reconfigure GitLab:
+
+   ```shell
+   sudo gitlab-ctl reconfigure
+   ```
+
+1. Confirm that GitLab Mattermost is reachable at `https://mattermost.example.com` and authorized to connect to GitLab. Authorizing Mattermost with GitLab allows users to use GitLab as an SSO provider.
+
+The Omnibus GitLab package attempts to automatically authorize GitLab Mattermost with GitLab if the applications are running on the same server.
+
+Automatic authorization requires access to the GitLab database. If the GitLab database is not available
+you will need to manually authorize GitLab Mattermost for access to GitLab using the process described in the [Authorize GitLab Mattermost section](#authorize-gitlab-mattermost).
+
+## Configuring Mattermost
+
+Starting in GitLab 11.0, Mattermost can be configured using the Mattermost System Console. An extensive list of
+Mattermost settings and where they can be set is available [in the Mattermost documentation](https://docs.mattermost.com/administration/config-settings.html).
+
+While using the System Console is recommended, you can also configure Mattermost using one of the following options:
+
+1. Edit the Mattermost configuration directly through `/var/opt/gitlab/mattermost/config.json`.
+1. Specify environment variables used to run Mattermost by changing the `mattermost['env']` setting in `gitlab.rb`. Any settings configured in this way will be disabled from the System Console and cannot be changed without restarting Mattermost.
+
+## Running GitLab Mattermost with HTTPS
+
+Place the SSL certificate and SSL certificate key inside `/etc/gitlab/ssl`. If the directory doesn't exist, create it:
+
+```shell
+sudo mkdir -p /etc/gitlab/ssl
+sudo chmod 755 /etc/gitlab/ssl
+sudo cp mattermost.gitlab.example.key mattermost.gitlab.example.crt /etc/gitlab/ssl/
+```
+
+In `/etc/gitlab/gitlab.rb` specify the following configuration:
+
+```ruby
+mattermost_external_url 'https://mattermost.gitlab.example'
+mattermost_nginx['redirect_http_to_https'] = true
+```
+
+If you haven't named your certificate and key `mattermost.gitlab.example.crt`
+and `mattermost.gitlab.example.key` then you'll need to also add the full paths
+as shown below.
+
+```ruby
+mattermost_nginx['ssl_certificate'] = "/etc/gitlab/ssl/mattermost-nginx.crt"
+mattermost_nginx['ssl_certificate_key'] = "/etc/gitlab/ssl/mattermost-nginx.key"
+```
+
+where `mattermost-nginx.crt` and `mattermost-nginx.key` are SSL cert and key, respectively.
+
+Once the configuration is set, run `sudo gitlab-ctl reconfigure` to apply the changes.
+
+## Running GitLab Mattermost on its own server
+
+If you want to run GitLab and GitLab Mattermost on two separate servers the GitLab services will still be set up on your GitLab Mattermost server, but they will not accept user requests or
+consume system resources. You can use the following settings and configuration details on the GitLab Mattermost server to effectively disable the GitLab service bundled into the Omnibus package.
+
+```ruby
+mattermost_external_url 'http://mattermost.example.com'
+
+# Shut down GitLab services on the Mattermost server
+gitlab_rails['enable'] = false
+redis['enable'] = false
+postgres_exporter['enable'] = false
+```
+
+Then follow the appropriate steps in the [Authorize GitLab Mattermost section](#authorize-gitlab-mattermost). Last, to enable
+integrations with GitLab add the following on the GitLab Server:
+
+```ruby
+gitlab_rails['mattermost_host'] = "https://mattermost.example.com"
+```
+
+By default GitLab Mattermost requires all users to sign up with GitLab and disables the sign-up by email option. See Mattermost [documentation on GitLab SSO](https://docs.mattermost.com/deployment/sso-gitlab.html).
+
+## Manually (re)authorizing GitLab Mattermost with GitLab
+
+### Reauthorize GitLab Mattermost
+
+To reauthorize GitLab Mattermost, you first need to revoke the existing
+authorization. This can be done in the **Settings > Applications** area of GitLab. Then follow the steps below to complete authorization.
+
+### Authorize GitLab Mattermost
+
+Navigate to the **Settings > Applications** area in GitLab. Create a new application and for the **Redirect URI** use the following (replace `http` with `https` if you use HTTPS):
+
+```plaintext
+http://mattermost.example.com/signup/gitlab/complete
+http://mattermost.example.com/login/gitlab/complete
+```
+
+Note that you do not need to select any options under **Scopes**. Choose **Save application**.
+
+Once the application is created you will be provided with an `Application ID` and `Secret`. One other piece of information needed is the URL of GitLab instance.
+Return to the server running GitLab Mattermost and edit the `/etc/gitlab/gitlab.rb` configuration file as follows using the values you received above:
+
+```ruby
+mattermost['gitlab_enable'] = true
+mattermost['gitlab_id'] = "12345656"
+mattermost['gitlab_secret'] = "123456789"
+mattermost['gitlab_scope'] = ""
+mattermost['gitlab_auth_endpoint'] = "http://gitlab.example.com/oauth/authorize"
+mattermost['gitlab_token_endpoint'] = "http://gitlab.example.com/oauth/token"
+mattermost['gitlab_user_api_endpoint'] = "http://gitlab.example.com/api/v4/user"
+```
+
+Save the changes and then run `sudo gitlab-ctl reconfigure`. If there are no errors your GitLab and GitLab Mattermost should be configured correctly.
+
+### Specify numeric user and group identifiers
+
+Omnibus GitLab creates a user and group `mattermost`. You can specify the
+numeric identifiers for these users in `/etc/gitlab/gitlab.rb` as follows:
+
+```ruby
+mattermost['uid'] = 1234
+mattermost['gid'] = 1234
+```
+
+Run `sudo gitlab-ctl reconfigure` to apply the changes.
+
+### Setting custom environment variables
+
+If necessary you can set custom environment variables to be used by Mattermost
+via `/etc/gitlab/gitlab.rb`. This can be useful if the Mattermost server
+is operated behind a corporate internet proxy. In `/etc/gitlab/gitlab.rb`
+supply a `mattermost['env']` with a hash value. For example:
+
+```ruby
+mattermost['env'] = {"HTTP_PROXY" => "my_proxy", "HTTPS_PROXY" => "my_proxy", "NO_PROXY" => "my_no_proxy"}
+```
+
+Run `sudo gitlab-ctl reconfigure` to apply the changes.
+
+### Connecting to the bundled PostgreSQL database
+
+If you need to connect to the bundled PostgreSQL database and are using the default Omnibus GitLab database configuration, you can connect as
+the PostgreSQL superuser:
+
+```shell
+sudo gitlab-psql -d mattermost_production
+```
+
+### Back up GitLab Mattermost
+
+GitLab Mattermost is not included in the regular [Omnibus GitLab backup](../../raketasks/backup_restore.md#back-up-gitlab) Rake task.
+
+The general Mattermost [backup and disaster recovery](https://docs.mattermost.com/deploy/backup-disaster-recovery.html) documentation can be used as a guide
+on what needs to be backed up.
+
+#### Back up the bundled PostgreSQL database
+
+If you need to back up the bundled PostgreSQL database and are using the default Omnibus GitLab database configuration, you can back up using this command:
+
+```shell
+sudo -i -u gitlab-psql -- /opt/gitlab/embedded/bin/pg_dump -h /var/opt/gitlab/postgresql mattermost_production | gzip > mattermost_dbdump_$(date --rfc-3339=date).sql.gz
+```
+
+#### Back up the `data` directory and `config.json`
+
+Mattermost has a `data` directory and `config.json` file that will need to be backed up as well:
+
+```shell
+sudo tar -zcvf mattermost_data_$(date --rfc-3339=date).gz -C /var/opt/gitlab/mattermost data config.json
+```
+
+### Restore GitLab Mattermost
+
+If you have previously [created a backup of GitLab Mattermost](#back-up-gitlab-mattermost), you can run the following commands to restore it:
+
+```shell
+# Stop Mattermost so we don't have any open database connections
+sudo gitlab-ctl stop mattermost
+
+# Drop the Mattermost database
+sudo -u gitlab-psql /opt/gitlab/embedded/bin/dropdb -U gitlab-psql -h /var/opt/gitlab/postgresql -p 5432 mattermost_production
+
+# Create the Mattermost database
+sudo -u gitlab-psql /opt/gitlab/embedded/bin/createdb -U gitlab-psql -h /var/opt/gitlab/postgresql -p 5432 mattermost_production
+
+# Perform the database restore
+# Replace /tmp/mattermost_dbdump_2021-08-05.sql.gz with your backup
+sudo -u mattermost sh -c "zcat /tmp/mattermost_dbdump_2021-08-05.sql.gz | /opt/gitlab/embedded/bin/psql -U gitlab_mattermost -h /var/opt/gitlab/postgresql -p 5432 mattermost_production"
+
+# Restore the data directory and config.json
+# Replace /tmp/mattermost_data_2021-08-09.gz with your backup
+sudo tar -xzvf /tmp/mattermost_data_2021-08-09.gz -C /var/opt/gitlab/mattermost
+
+# Fix permissions if required
+sudo chown -R mattermost:mattermost /var/opt/gitlab/mattermost/data
+sudo chown mattermost:mattermost /var/opt/gitlab/mattermost/config.json
+
+# Start Mattermost
+sudo gitlab-ctl start mattermost
+```
+
+### Mattermost Command Line Tools (CLI)
+
+NOTE:
+This CLI will be replaced in a future release with the new [`mmctl` Command Line Tool](https://docs.mattermost.com/administration/mmctl-cli-tool.html).
+
+To use the [Mattermost Command Line Tools (CLI)](https://docs.mattermost.com/administration/command-line-tools.html), ensure that you are in the `/opt/gitlab/embedded/service/mattermost` directory when you run the CLI commands and that you specify the location of the configuration file. The executable is `/opt/gitlab/embedded/bin/mattermost`.
+
+```shell
+cd /opt/gitlab/embedded/service/mattermost
+
+sudo /opt/gitlab/embedded/bin/chpst -e /opt/gitlab/etc/mattermost/env -P -U mattermost:mattermost -u mattermost:mattermost /opt/gitlab/embedded/bin/mattermost --config=/var/opt/gitlab/mattermost/config.json version
+```
+
+Until [#4745](https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/4745) has been implemented, the command requires quite of bit typing and is hard to remember, so let's make a bash/zsh alias to make it a bit easier to remember. Add the following to your `~/.bashrc` or `~/.zshrc` file:
+
+```shell
+alias mattermost-cli="cd /opt/gitlab/embedded/service/mattermost && sudo /opt/gitlab/embedded/bin/chpst -e /opt/gitlab/etc/mattermost/env -P -U mattermost:mattermost -u mattermost:mattermost /opt/gitlab/embedded/bin/mattermost --config=/var/opt/gitlab/mattermost/config.json $1"
+```
+
+Then source `~/.zshrc` or `~/.bashrc` with `source ~/.zshrc` or `source ~/.bashrc`.
+
+If successful, you can now run any Mattermost CLI command with your new shell alias `mattermost-cli`:
+
+```shell
+$ mattermost-cli version
+
+[sudo] password for username:
+{"level":"info","ts":1569614421.9058893,"caller":"utils/i18n.go:83","msg":"Loaded system translations for 'en' from '/opt/gitlab/embedded/service/mattermost/i18n/en.json'"}
+{"level":"info","ts":1569614421.9062793,"caller":"app/server_app_adapters.go:58","msg":"Server is initializing..."}
+{"level":"info","ts":1569614421.90976,"caller":"sqlstore/supplier.go:223","msg":"Pinging SQL master database"}
+{"level":"info","ts":1569614422.0515099,"caller":"mlog/log.go:165","msg":"Starting up plugins"}
+{"level":"info","ts":1569614422.0515954,"caller":"app/plugin.go:193","msg":"Syncing plugins from the file store"}
+{"level":"info","ts":1569614422.086005,"caller":"app/plugin.go:228","msg":"Found no files in plugins file store"}
+{"level":"info","ts":1569614423.9337213,"caller":"sqlstore/post_store.go:1301","msg":"Post.Message supports at most 16383 characters (65535 bytes)"}
+{"level":"error","ts":1569614425.6317747,"caller":"go-plugin/stream.go:15","msg":" call to OnConfigurationChange failed, error: Must have a GitLab oauth client id","plugin_id":"com.github.manland.mattermost-plugin-gitlab","source":"plugin_stderr"}
+{"level":"info","ts":1569614425.6875598,"caller":"mlog/sugar.go:19","msg":"Ensuring Surveybot exists","plugin_id":"com.mattermost.nps"}
+{"level":"info","ts":1569614425.6953356,"caller":"app/server.go:216","msg":"Current version is 5.14.0 (5.14.2/Fri Aug 30 20:20:48 UTC 2019/817ee89711bf26d33f840ce7f59fba14da1ed168/none)"}
+{"level":"info","ts":1569614425.6953766,"caller":"app/server.go:217","msg":"Enterprise Enabled: false"}
+{"level":"info","ts":1569614425.6954057,"caller":"app/server.go:219","msg":"Current working directory is /opt/gitlab/embedded/service/mattermost/i18n"}
+{"level":"info","ts":1569614425.6954265,"caller":"app/server.go:220","msg":"Loaded config","source":"file:///var/opt/gitlab/mattermost/config.json"}
+Version: 5.14.0
+Build Number: 5.14.2
+Build Date: Fri Aug 30 20:20:48 UTC 2019
+Build Hash: 817ee89711bf26d33f840ce7f59fba14da1ed168
+Build Enterprise Ready: false
+DB Version: 5.14.0
+```
+
+For more details see [Mattermost Command Line Tools (CLI)](https://docs.mattermost.com/administration/command-line-tools.html) and the [Troubleshooting Mattermost CLI](#troubleshooting-the-mattermost-cli) below.
+
+## Configuring GitLab and Mattermost integrations
+
+As of 12.3, the Mattermost GitLab plugin is shipped with Omnibus GitLab: [Mattermost Plugin for GitLab documentation](https://github.com/mattermost/mattermost-plugin-gitlab).
+
+You can use the plugin to subscribe Mattermost to receive notifications about issues, merge requests, and pull requests as well as personal notifications regarding merge request reviews, unread messages, and task assignments. If you want to use slash commands to perform actions
+such as creating and viewing issues, or to trigger deployments use GitLab [Mattermost slash commands](../../user/project/integrations/mattermost_slash_commands.md).
+
+The plugin and slash commands can be used together or individually.
+
+## Email Notifications
+
+### Setting up SMTP for GitLab Mattermost
+
+These settings are configured through the Mattermost System Console by the System Administrator.
+On the **Environment > SMTP** tab of the **System Console**, you can enter the SMTP credentials given by your SMTP provider, or `127.0.0.1` and port `25` to use `sendmail`. More information on the specific settings
+that are needed is available in the [Mattermost documentation](https://docs.mattermost.com/install/smtp-email-setup.html).
+
+These settings can also be configured in `/var/opt/gitlab/mattermost/config.json`.
+
+### Email batching
+
+Enabling this feature allows users to control how often they receive email notifications.
+
+Email batching can be enabled in the Mattermost **System Console** by going to the **Environment > SMTP** tab, and setting the **Enable Email Batching** setting to **True**.
+
+This setting can also be configured in `/var/opt/gitlab/mattermost/config.json`.
+
+## Upgrading GitLab Mattermost
+
+Below is a list of Mattermost versions for GitLab 11.10 and later:
+
+| GitLab Version  | Mattermost Version |
+| :------------ |:----------------|
+| 11.11 | 5.10 |
+| 12.0 | 5.11 |
+| 12.1 | 5.12 |
+| 12.2 | 5.13 |
+| 12.3 | 5.14 |
+| 12.4 | 5.15 |
+| 12.5 | 5.16 |
+| 12.6 | 5.17 |
+| 12.7 | 5.17 |
+| 12.8 | 5.19 |
+| 12.9 | 5.20 |
+| 12.10 | 5.21 |
+| 13.0 | 5.22 |
+| 13.1 | 5.23 |
+| 13.2 | 5.24 |
+| 13.3 | 5.25 |
+| 13.4 | 5.26 |
+| 13.5 | 5.27 |
+| 13.6 | 5.28 |
+| 13.7 | 5.29 |
+| 13.8 | 5.30 |
+| 13.9 | 5.31 |
+| 13.10 | 5.32 |
+| 13.11 | 5.33 |
+| 13.12 | 5.34 |
+| 14.0 | 5.35 |
+| 14.1 | 5.36 |
+| 14.2 | 5.37 |
+| 14.3 | 5.38 |
+
+NOTE:
+When upgrading the Mattermost version, it is essential to check the
+[Important Upgrade Notes](https://docs.mattermost.com/administration/important-upgrade-notes.html)
+for Mattermost to address any changes or migrations that need to be performed.
+
+Starting with GitLab 11.0, GitLab Mattermost can be upgraded through the regular Omnibus GitLab update process. When upgrading previous versions of
+GitLab that process can only be used if Mattermost configuration settings have not been changed outside of GitLab (i.e., no changes to Mattermost's `config.json`
+file have been made, either directly or via the Mattermost **System Console** which saves back changes to `config.json`.)
+
+If you are upgrading to at least GitLab 11.0 or have only configured Mattermost using `gitlab.rb`, you can upgrade GitLab using Omnibus and then run `gitlab-ctl reconfigure` to upgrade GitLab Mattermost to the latest version.
+
+If this is not the case, there are two options:
+
+1. Update [`gitlab.rb`](https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-config-template/gitlab.rb.template#L706)
+   with the changes done to `config.json`. This might require adding some parameters as not all
+   settings in `config.json` are available in `gitlab.rb`. Once complete, Omnibus GitLab should be
+   able to upgrade GitLab Mattermost from one version to the next.
+1. Migrate Mattermost outside of the directory controlled by Omnibus GitLab so it can be administered
+   and upgraded independently. Follow the [Mattermost Migration Guide](https://docs.mattermost.com/administration/migrating.html)
+   to move your Mattermost configuration settings and data to another directory or server independent
+   from Omnibus GitLab.
+
+For a complete list of upgrade notices and special considerations for older versions, see the [Mattermost documentation](https://docs.mattermost.com/administration/important-upgrade-notes.html).
+
+## Upgrading GitLab Mattermost from versions prior to 11.0
+
+With version 11.0, GitLab introduced breaking changes which affected Mattermost configuration.
+In versions prior to GitLab 11.0 all
+Mattermost-related settings were configurable from the `gitlab.rb` file, which
+generated the Mattermost `config.json` file. However, Mattermost also
+permitted configuration via its System Console. This configuration ended up in
+the same `config.json` file, which resulted in changes made via the System Console being
+overwritten when users ran `gitlab-ctl reconfigure`.
+
+To resolve this problem, `gitlab.rb` includes only the
+configuration necessary for GitLab<=>Mattermost integration in 11.0. GitLab no longer
+generates the `config.json` file, and instead passes limited configuration settings via environment variables.
+
+The settings that continue to be supported in `gitlab.rb` can be found in
+[`gitlab.rb.template`](https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-config-template/gitlab.rb.template).
+
+From GitLab 11.0, other Mattermost settings can be configured through Mattermost's System Console,
+by editing `/var/opt/gitlab/mattermost/config.json`, or by using `mattermost['env']` in `gitlab.rb`.
+
+If you would like to keep configuring Mattermost using `gitlab.rb`, you can take the following actions
+in preparation for GitLab 11.0:
+
+1. Upgrade to version 10.x which supports the new `mattermost['env']` setting.
+1. Configure any settings not listed above through the `mattermost['env']` setting. Mattermost requires
+   environment variables to be provided in `MM_<CATEGORY>SETTINGS_<ATTRIBUTE>` format. Below is an example
+   of how to convert the old settings syntax to the new one.
+
+The following settings in `gitlab.rb`:
+
+```ruby
+mattermost['service_maximum_login_attempts'] = 10
+mattermost['team_teammate_name_display'] = "full_name"
+mattermost['sql_max_idle_conns'] = 10
+mattermost['log_file_level'] = 'INFO'
+mattermost['email_batching_interval'] = 30
+mattermost['file_enable_file_attachments'] = true
+mattermost['ratelimit_memory_store_size'] = 10000
+mattermost['support_terms_of_service_link'] = "/static/help/terms.html"
+mattermost['privacy_show_email_address'] = true
+mattermost['localization_available_locales'] = "en,es,fr,ja,pt-BR"
+mattermost['webrtc_enable'] = false
+```
+
+Would translate to:
+
+```ruby
+mattermost['env'] = {
+                    'MM_SERVICESETTINGS_MAXIMUMLOGINATTEMPTS' => '10',
+                    'MM_TEAMSETTINGS_TEAMMATENAMEDISPLAY' => 'full_name',
+                    'MM_SQLSETTINGS_MAXIDLECONNS' => '10',
+                    'MM_LOGSETTINGS_FILELEVEL' => 'INFO',
+                    'MM_EMAILSETTINGS_BATCHINGINTERVAL' => '30',
+                    'MM_FILESETTINGS_ENABLEFILEATTACHMENTS' => 'true',
+                    'MM_RATELIMITSETTINGS_MEMORYSTORESIZE' => '10000',
+                    'MM_SUPPORTSETTINGS_TERMSOFSERVICELINK' => '/static/help/terms.html',
+                    'MM_PRIVACYSETTINGS_SHOWEMAILADDRESS' => 'true',
+                    'MM_LOCALIZATIONSETTINGS_AVAILABLELOCALES' => 'en,es,fr,ja,pt-BR',
+                    'MM_WEBRTCSETTINGS_ENABLE' => 'false'
+                    }
+```
+
+Refer to the [Mattermost Configuration Settings
+documentation](https://docs.mattermost.com/administration/config-settings.html)
+for details about categories, configuration values, etc.
+
+There are a few exceptions to this rule:
+
+1. `ServiceSettings.ListenAddress` configuration of Mattermost is configured
+   by `mattermost['service_address']` and `mattermost['service_port']` settings.
+1. Configuration settings named in an inconsistent way are given in the
+   following table. Use these mappings when converting them to environment
+   variables.
+
+|`gitlab.rb` configuration|Environment variable|
+|---|---|
+|`mattermost['service_lets_encrypt_cert_cache_file']`|`MM_SERVICESETTINGS_LETSENCRYPTCERTIFICATECACHEFILE`|
+|`mattermost['service_user_access_tokens']`|`MM_SERVICESETTINGS_ENABLEUSERACCESSTOKENS`|
+|`mattermost['log_console_enable']`|`MM_LOGSETTINGS_ENABLECONSOLE`|
+|`mattermost['email_enable_batching']`|`MM_EMAILSETTINGS_ENABLEEMAILBATCHING`|
+|`mattermost['email_batching_buffer_size']`|`MM_EMAILSETTINGS_EMAILBATCHINGBUFFERSIZE`|
+|`mattermost['email_batching_interval']`|`MM_EMAILSETTINGS_EMAILBATCHINGINTERVAL`|
+|`mattermost['email_smtp_auth']`|`MM_EMAILSETTINGS_ENABLESMTPAUTH`|
+|`mattermost['email_notification_content_type']`|`MM_EMAILSETTINGS_NOTIFICATIONCONTENTTYPE`|
+|`mattermost['ratelimit_enable_ratelimiter']`|`MM_RATELIMITSETTINGS_ENABLE`|
+|`mattermost['support_email']`|`MM_SUPPORTSETTINGS_SUPPORTEMAIL`|
+|`mattermost['localization_server_locale']`|`MM_LOCALIZATIONSETTINGS_DEFAULTSERVERLOCALE`|
+|`mattermost['localization_client_locale']`|`MM_LOCALIZATIONSETTINGS_DEFAULTCLIENTLOCALE`|
+|`mattermost['webrtc_gateway_stun_uri']`|`MM_WEBRTCSETTINGS_STUN_URI`|
+|`mattermost['webrtc_gateway_turn_uri']`|`MM_WEBRTCSETTINGS_TURN_URI`|
+|`mattermost['webrtc_gateway_turn_username']`|`MM_WEBRTCSETTINGS_TURN_USERNAME`|
+|`mattermost['webrtc_gateway_turn_sharedkey']`|`MM_WEBRTCSETTINGS_TURN_SHAREDKEY`|
+
+NOTE:
+GitLab 11.0 no longer generates `config.json` file from the configuration specified
+in `gitlab.rb`. Users are responsible for managing this file which can be done via the
+Mattermost System Console or manually.
+If a configuration setting is specified via both the `gitlab.rb` (as an environment variable)
+and `config.json` files, the environment variable gets precedence.
+
+If you encounter any issues [visit the GitLab Mattermost troubleshooting forum](https://forum.mattermost.org/t/upgrading-to-gitlab-mattermost-in-gitlab-8-9/1735) and share any relevant portions of `mattermost.log` along with the step at which you encountered issues.
+
+### Upgrading GitLab Mattermost outside of GitLab
+
+If you choose to upgrade Mattermost outside of the Omnibus GitLab automation, [follow this guide](https://docs.mattermost.com/administration/upgrade.html).
+
+## OAuth2 sequence diagram
+
+The following image is a sequence diagram for how GitLab works as an OAuth2
+provider for Mattermost. You can use this to troubleshoot errors
+in getting the integration to work:
+
+![sequence diagram](img/gitlab-mattermost.png)
+
+## Troubleshooting the Mattermost CLI
+
+### Failed to ping DB retrying in 10 seconds err=dial tcp: lookup dockerhost: no such host
+
+As of version 11.0, majority of the Mattermost settings are now configured via environmental variables. The error is mainly due to the database connection string being commented out in `gitlab.rb` and the database connection settings being set in environmental variables. Additionally, the connection string in the `gitlab.rb` is for MySQL which is no longer supported as of 12.1.
+
+You can fix this by setting up a `mattermost-cli` [shell alias](#mattermost-command-line-tools-cli).
+
+## Community support resources
+
+For help and support around your GitLab Mattermost deployment please see:
+
+- [Troubleshooting Forum](https://forum.mattermost.org/t/how-to-use-the-troubleshooting-forum/150) for configuration questions and issues.
+- [Troubleshooting FAQ](https://docs.mattermost.com/install/troubleshooting.html).
+- [Mattermost GitLab Issues Support Handbook](https://docs.mattermost.com/process/support.html?highlight=omnibus#gitlab-issues).
+- [GitLab Mattermost issue tracker](https://gitlab.com/gitlab-org/gitlab-mattermost/-/issues) for verified bugs with repro steps.