Move container_registry user docs to new location

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+# GitLab Container Registry
+
+> [Introduced][ce-4040] in GitLab 8.8.
+
+---
+
+> **Note**
+Docker Registry manifest `v1` support was added in GitLab 8.9 to support Docker
+versions earlier than 1.10.
+>
+This document is about the user guide. To learn how to enable GitLab Container
+Registry across your GitLab instance, visit the
+[administrator documentation](../../administration/container_registry.md).
+
+With the Docker Container Registry integrated into GitLab, every project can
+have its own space to store its Docker images.
+
+You can read more about Docker Registry at https://docs.docker.com/registry/introduction/.
+
+---
+
+## Enable the Container Registry for your project
+
+1. First, ask your system administrator to enable GitLab Container Registry
+   following the [administration documentation](../../administration/container_registry.md).
+   If you are using GitLab.com, this is enabled by default so you can start using
+   the Registry immediately.
+
+1. Go to your project's settings and enable the **Container Registry** feature
+   on your project. For new projects this might be enabled by default. For
+   existing projects (prior GitLab 8.8), you will have to explicitly enable it.
+
+    ![Enable Container Registry](img/container_registry_enable.png)
+
+1. Hit **Save changes** for the changes to take effect. You should now be able
+   to see the **Registry** link in the project menu.
+
+    ![Container Registry tab](img/container_registry_tab.png)
+
+## Build and push images
+
+If you visit the **Registry** link under your project's menu, you can see the
+explicit instructions to login to the Container Registry using your GitLab
+credentials.
+
+For example if the Registry's URL is `registry.example.com`, the you should be
+able to login with:
+
+```
+docker login registry.example.com
+```
+
+Building and publishing images should be a straightforward process. Just make
+sure that you are using the Registry URL with the namespace and project name
+that is hosted on GitLab:
+
+```
+docker build -t registry.example.com/group/project .
+docker push registry.example.com/group/project
+```
+
+Your image will be named after the following scheme:
+
+```
+<registry URL>/<namespace>/<project>
+```
+
+As such, the name of the image is unique, but you can differentiate the images
+using tags.
+
+## Use images from GitLab Container Registry
+
+To download and run a container from images hosted in GitLab Container Registry,
+use `docker run`:
+
+```
+docker run [options] registry.example.com/group/project [arguments]
+```
+
+For more information on running Docker containers, visit the
+[Docker documentation][docker-docs].
+
+## Control Container Registry from within GitLab
+
+GitLab offers a simple Container Registry management panel. Go to your project
+and click **Registry** in the project menu.
+
+This view will show you all tags in your project and will easily allow you to
+delete them.
+
+![Container Registry panel](img/container_registry_panel.png)
+
+## Build and push images using GitLab CI
+
+> **Note:**
+This feature requires GitLab 8.8 and GitLab Runner 1.2.
+
+Make sure that your GitLab Runner is configured to allow building Docker images by
+following the [Using Docker Build](../ci/docker/using_docker_build.md)
+and [Using the GitLab Container Registry documentation](../ci/docker/using_docker_build.md#using-the-gitlab-container-registry).
+
+## Limitations
+
+In order to use a container image from your private project as an `image:` in
+your `.gitlab-ci.yml`, you have to follow the
+[Using a private Docker Registry][private-docker]
+documentation. This workflow will be simplified in the future.
+
+## Troubleshooting the GitLab Container Registry
+
+### Basic Troubleshooting
+
+1. Check to make sure that the system clock on your Docker client and GitLab server have
+   been synchronized (e.g. via NTP).
+
+2. If you are using an S3-backed Registry, double check that the IAM
+   permissions and the S3 credentials (including region) are correct. See [the
+   sample IAM policy](https://docs.docker.com/registry/storage-drivers/s3/)
+   for more details.
+
+3. Check the Registry logs (e.g. `/var/log/gitlab/registry/current`) and the GitLab production logs
+   for errors (e.g. `/var/log/gitlab/gitlab-rails/production.log`). You may be able to find clues
+   there.
+
+### Advanced Troubleshooting
+
+>**NOTE:** The following section is only recommended for experts.
+
+Sometimes it's not obvious what is wrong, and you may need to dive deeper into
+the communication between the Docker client and the Registry to find out
+what's wrong. We will use a concrete example in the past to illustrate how to
+diagnose a problem with the S3 setup.
+
+#### Unexpected 403 error during push
+
+A user attempted to enable an S3-backed Registry. The `docker login` step went
+fine. However, when pushing an image, the output showed:
+
+```
+The push refers to a repository [s3-testing.myregistry.com:4567/root/docker-test]
+dc5e59c14160: Pushing [==================================================>] 14.85 kB
+03c20c1a019a: Pushing [==================================================>] 2.048 kB
+a08f14ef632e: Pushing [==================================================>] 2.048 kB
+228950524c88: Pushing 2.048 kB
+6a8ecde4cc03: Pushing [==>                                                ] 9.901 MB/205.7 MB
+5f70bf18a086: Pushing 1.024 kB
+737f40e80b7f: Waiting
+82b57dbc5385: Waiting
+19429b698a22: Waiting
+9436069b92a3: Waiting
+error parsing HTTP 403 response body: unexpected end of JSON input: ""
+```
+
+This error is ambiguous, as it's not clear whether the 403 is coming from the
+GitLab Rails application, the Docker Registry, or something else. In this
+case, since we know that since the login succeeded, we probably need to look
+at the communication between the client and the Registry.
+
+The REST API between the Docker client and Registry is [described
+here](https://docs.docker.com/registry/spec/api/). Normally, one would just
+use Wireshark or tcpdump to capture the traffic and see where things went
+wrong.  However, since all communication between Docker clients and servers
+are done over HTTPS, it's a bit difficult to decrypt the traffic quickly even
+if you know the private key. What can we do instead?
+
+One way would be to disable HTTPS by setting up an [insecure
+Registry](https://docs.docker.com/registry/insecure/). This could introduce a
+security hole and is only recommended for local testing. If you have a
+production system and can't or don't want to do this, there is another way:
+use mitmproxy, which stands for Man-in-the-Middle Proxy.
+
+#### mitmproxy
+
+[mitmproxy](https://mitmproxy.org/) allows you to place a proxy between your
+client and server to inspect all traffic. One wrinkle is that your system
+needs to trust the mitmproxy SSL certificates for this to work.
+
+The following installation instructions assume you are running Ubuntu:
+
+1. Install mitmproxy (see http://docs.mitmproxy.org/en/stable/install.html)
+1. Run `mitmproxy --port 9000` to generate its certificates.
+   Enter <kbd>CTRL</kbd>-<kbd>C</kbd> to quit.
+1. Install the certificate from `~/.mitmproxy` to your system:
+
+    ```sh
+    sudo cp ~/.mitmproxy/mitmproxy-ca-cert.pem /usr/local/share/ca-certificates/mitmproxy-ca-cert.crt
+    sudo update-ca-certificates
+    ```
+
+If successful, the output should indicate that a certificate was added:
+
+```sh
+Updating certificates in /etc/ssl/certs... 1 added, 0 removed; done.
+Running hooks in /etc/ca-certificates/update.d....done.
+```
+
+To verify that the certificates are properly installed, run:
+
+```sh
+mitmproxy --port 9000
+```
+
+This will run mitmproxy on port `9000`. In another window, run:
+
+```sh
+curl --proxy http://localhost:9000 https://httpbin.org/status/200
+```
+
+If everything is setup correctly, you will see information on the mitmproxy window and
+no errors from the curl commands.
+
+#### Running the Docker daemon with a proxy
+
+For Docker to connect through a proxy, you must start the Docker daemon with the
+proper environment variables. The easiest way is to shutdown Docker (e.g. `sudo initctl stop docker`)
+and then run Docker by hand. As root, run:
+
+```sh
+export HTTP_PROXY="http://localhost:9000"
+export HTTPS_PROXY="https://localhost:9000"
+docker daemon --debug
+```
+
+This will launch the Docker daemon and proxy all connections through mitmproxy.
+
+#### Running the Docker client
+
+Now that we have mitmproxy and Docker running, we can attempt to login and push
+a container image. You may need to run as root to do this. For example:
+
+```sh
+docker login s3-testing.myregistry.com:4567
+docker push s3-testing.myregistry.com:4567/root/docker-test
+```
+
+In the example above, we see the following trace on the mitmproxy window:
+
+![mitmproxy output from Docker](img/mitmproxy-docker.png)
+
+The above image shows:
+
+* The initial PUT requests went through fine with a 201 status code.
+* The 201 redirected the client to the S3 bucket.
+* The HEAD request to the AWS bucket reported a 403 Unauthorized.
+
+What does this mean? This strongly suggests that the S3 user does not have the right
+[permissions to perform a HEAD request](http://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectHEAD.html).
+The solution: check the [IAM permissions again](https://docs.docker.com/registry/storage-drivers/s3/).
+Once the right permissions were set, the error will go away.
+
+[ce-4040]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4040
+[docker-docs]: https://docs.docker.com/engine/userguide/intro/
+[private-docker]: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/blob/master/docs/configuration/advanced-configuration.md#using-a-private-docker-registry
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