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% DOCKERFILE(5) Docker User Manuals
% Zac Dover
% May 2014
# NAME
Dockerfile - automate the steps of creating a Docker image
# INTRODUCTION
The **Dockerfile** is a configuration file that automates the steps of creating
a Docker image. It is similar to a Makefile. Docker reads instructions from the
**Dockerfile** to automate the steps otherwise performed manually to create an
image. To build an image, create a file called **Dockerfile**. The
**Dockerfile** describes the steps taken to assemble the image. When the
**Dockerfile** has been created, call the **docker build** command, using the
path of directory that contains **Dockerfile** as the argument.
# SYNOPSIS
INSTRUCTION arguments
For example:
FROM image
# DESCRIPTION
A Dockerfile is a file that automates the steps of creating a Docker image.
A Dockerfile is similar to a Makefile.
# USAGE
**sudo docker build .**
-- runs the steps and commits them, building a final image
The path to the source repository defines where to find the context of the
build. The build is run by the docker daemon, not the CLI. The whole
context must be transferred to the daemon. The Docker CLI reports
"Sending build context to Docker daemon" when the context is sent to the daemon.
**sudo docker build -t repository/tag .**
-- specifies a repository and tag at which to save the new image if the build
succeeds. The Docker daemon runs the steps one-by-one, committing the result
to a new image if necessary before finally outputting the ID of the new
image. The Docker daemon automatically cleans up the context it is given.
Docker re-uses intermediate images whenever possible. This significantly
accelerates the *docker build* process.
# FORMAT
**FROM image**
or
**FROM image:tag**
-- The FROM instruction sets the base image for subsequent instructions. A
valid Dockerfile must have FROM as its first instruction. The image can be any
valid image. It is easy to start by pulling an image from the public
repositories.
-- FROM must be he first non-comment instruction in Dockerfile.
-- FROM may appear multiple times within a single Dockerfile in order to create
multiple images. Make a note of the last image id output by the commit before
each new FROM command.
-- If no tag is given to the FROM instruction, latest is assumed. If the used
tag does not exist, an error is returned.
**MAINTAINER**
--The MAINTAINER instruction sets the Author field for the generated images.
**RUN**
--RUN has two forms:
**RUN <command>**
-- (the command is run in a shell - /bin/sh -c)
**RUN ["executable", "param1", "param2"]**
--The above is executable form.
--The RUN instruction executes any commands in a new layer on top of the
current image and commits the results. The committed image is used for the next
step in Dockerfile.
--Layering RUN instructions and generating commits conforms to the core
concepts of Docker where commits are cheap and containers can be created from
any point in the history of an image. This is similar to source control. The
exec form makes it possible to avoid shell string munging. The exec form makes
it possible to RUN commands using a base image that does not contain /bin/sh.
**CMD**
--CMD has three forms:
**CMD ["executable", "param1", "param2"]** This is the preferred form, the
exec form.
**CMD ["param1", "param2"]** This command provides default parameters to
ENTRYPOINT)
**CMD command param1 param2** This command is run as a shell.
--There can be only one CMD in a Dockerfile. If more than one CMD is listed, only
the last CMD takes effect.
The main purpose of a CMD is to provide defaults for an executing container.
These defaults may include an executable, or they can omit the executable. If
they omit the executable, an ENTRYPOINT must be specified.
When used in the shell or exec formats, the CMD instruction sets the command to
be executed when running the image.
If you use the shell form of of the CMD, the <command> executes in /bin/sh -c:
**FROM ubuntu**
**CMD echo "This is a test." | wc -**
If you run <command> wihtout a shell, then you must express the command as a
JSON arry and give the full path to the executable. This array form is the
preferred form of CMD. All additional parameters must be individually expressed
as strings in the array:
**FROM ubuntu**
**CMD ["/usr/bin/wc","--help"]**
To make the container run the same executable every time, use ENTRYPOINT in
combination with CMD.
If the user specifies arguments to docker run, the specified commands override
the default in CMD.
Do not confuse **RUN** with **CMD**. RUN runs a command and commits the result. CMD
executes nothing at build time, but specifies the intended command for the
image.
**EXPOSE**
--**EXPOSE <port> [<port>...]**
The **EXPOSE** instruction informs Docker that the container listens on the
specified network ports at runtime. Docker uses this information to
interconnect containers using links, and to set up port redirection on the host
system.
**ENV**
--**ENV <key> <value>**
The ENV instruction sets the environment variable <key> to
the value <value>. This value is passed to all future RUN instructions. This is
functionally equivalent to prefixing the command with **<key>=<value>**. The
environment variables that are set with ENV persist when a container is run
from the resulting image. Use docker inspect to inspect these values, and
change them using docker run **--env <key>=<value>.**
Note that setting Setting **ENV DEBIAN_FRONTEND noninteractive** may cause
unintended consequences, because it will persist when the container is run
interactively, as with the following command: **docker run -t -i image bash**
**ADD**
--**ADD <src> <dest>** The ADD instruction copies new files from <src> and adds them
to the filesystem of the container at path <dest>. <src> must be the path to a
file or directory relative to the source directory that is being built (the
context of the build) or a remote file URL. <dest> is the absolute path to
which the source is copied inside the target container. All new files and
directories are created with mode 0755, with uid and gid 0.
**ENTRYPOINT**
--**ENTRYPOINT** has two forms: ENTRYPOINT ["executable", "param1", "param2"]
(This is like an exec, and is the preferred form.) ENTRYPOINT command param1
param2 (This is running as a shell.) An ENTRYPOINT helps you configure a
container that can be run as an executable. When you specify an ENTRYPOINT,
the whole container runs as if it was only that executable. The ENTRYPOINT
instruction adds an entry command that is not overwritten when arguments are
passed to docker run. This is different from the behavior of CMD. This allows
arguments to be passed to the entrypoint, for instance docker run <image> -d
passes the -d argument to the ENTRYPOINT. Specify parameters either in the
ENTRYPOINT JSON array (as in the preferred exec form above), or by using a CMD
statement. Parameters in the ENTRYPOINT are not overwritten by the docker run
arguments. Parameters specifies via CMD are overwritten by docker run
arguments. Specify a plain string for the ENTRYPOINT, and it will execute in
/bin/sh -c, like a CMD instruction:
FROM ubuntu
ENTRYPOINT wc -l -
This means that the Dockerfile's image always takes stdin as input (that's
what "-" means), and prints the number of lines (that's what "-l" means). To
make this optional but default, use a CMD:
FROM ubuntu
CMD ["-l", "-"]
ENTRYPOINT ["/usr/bin/wc"]
**VOLUME**
--**VOLUME ["/data"]**
The VOLUME instruction creates a mount point with the specified name and marks
it as holding externally-mounted volumes from the native host or from other
containers.
**USER**
-- **USER daemon**
The USER instruction sets the username or UID that is used when running the
image.
**WORKDIR**
-- **WORKDIR /path/to/workdir**
The WORKDIR instruction sets the working directory for the **RUN**, **CMD**, and **ENTRYPOINT** Dockerfile commands that follow it.
It can be used multiple times in a single Dockerfile. Relative paths are defined relative to the path of the previous **WORKDIR** instruction. For example:
**WORKDIR /a WORKDIR /b WORKDIR c RUN pwd**
In the above example, the output of the **pwd** command is **a/b/c**.
**ONBUILD**
-- **ONBUILD [INSTRUCTION]**
The ONBUILD instruction adds a trigger instruction to the image, which is
executed at a later time, when the image is used as the base for another
build. The trigger is executed in the context of the downstream build, as
if it had been inserted immediately after the FROM instruction in the
downstream Dockerfile. Any build instruction can be registered as a
trigger. This is useful if you are building an image to be
used as a base for building other images, for example an application build
environment or a daemon to be customized with a user-specific
configuration. For example, if your image is a reusable python
application builder, it requires application source code to be
added in a particular directory, and might require a build script
to be called after that. You can't just call ADD and RUN now, because
you don't yet have access to the application source code, and it
is different for each application build. Providing
application developers with a boilerplate Dockerfile to copy-paste
into their application is inefficient, error-prone, and
difficult to update because it mixes with application-specific code.
The solution is to use **ONBUILD** to register instructions in advance, to
run later, during the next build stage.
# HISTORY
*May 2014, Compiled by Zac Dover (zdover at redhat dot com) based on docker.io Dockerfile documentation.
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