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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
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 the 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
-- MAINTAINER sets the Author field for the generated images.
RUN
-- RUN has two forms:
# the command is run in a shell - /bin/sh -c
RUN <command>
# Executable form
RUN ["executable", "param1", "param2"]
-- 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.
Note that the exec form is parsed as a JSON array, which means that you must
use double-quotes (") around words not single-quotes (').
CMD
-- CMD has three forms:
# Executable form
CMD ["executable", "param1", "param2"]`
# Provide default arguments to ENTRYPOINT
CMD ["param1", "param2"]`
# the command is run in a shell - /bin/sh -c
CMD command param1 param2
-- 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 the CMD, the <command> executes in /bin/sh -c:
Note that the exec form is parsed as a JSON array, which means that you must
use double-quotes (") around words not single-quotes (').
FROM ubuntu
CMD echo "This is a test." | wc -
-- If you run command without a shell, then you must express the command as a
JSON array 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.
LABEL
-- LABEL <key>[=<value>] [<key>[=<value>] ...]
The LABEL instruction adds metadata to an image. A LABEL is a
key-value pair. To include spaces within a LABEL value, use quotes and
backslashes as you would in command-line parsing.
LABEL "com.example.vendor"="ACME Incorporated"
An image can have more than one label. To specify multiple labels, separate
each key-value pair by a space.
Labels are additive including LABELs in FROM images. As the system
encounters and then applies a new label, new keys override any previous
labels with identical keys.
To display an image's labels, use the docker inspect command.
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 to
the value <value>. This value is passed to all future
RUN, ENTRYPOINT, and CMD 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 "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 has two forms:
ADD <src> <dest>
# Required for paths with whitespace
ADD ["<src>", "<dest>"]
The ADD instruction copies new files, directories
or remote file URLs to the filesystem of the container at path <dest>.
Multiple <src> resources may be specified but if they are files or directories
then they must be relative to the source directory that is being built
(the context of the build). The <dest> is the absolute path, or path relative
to WORKDIR, into which the source is copied inside the target container.
All new files and directories are created with mode 0755 and with the uid
and gid of 0.
COPY
-- COPY has two forms:
COPY <src> <dest>
# Required for paths with whitespace
COPY ["<src>", "<dest>"]
The COPY instruction copies new files from <src> and
adds them to the filesystem of the container at path . The <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. The <dest> is an
absolute path, or a path relative to WORKDIR, into which the source will
be copied inside the target container. All new files and directories are
created with mode 0755 and with the uid and gid of 0.
ENTRYPOINT
-- ENTRYPOINT has two forms:
# executable form
ENTRYPOINT ["executable", "param1", "param2"]`
# run command in a shell - /bin/sh -c
ENTRYPOINT command param1 param2
-- 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.
WRKDIR
-- WORKDIR /path/to/workdir
The WORKDIR instruction sets the working directory for the RUN, CMD,
ENTRYPOINT, COPY and ADD 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 an image. The
trigger is executed at a later time, when the image is used as the base for
another build. Docker executes the trigger in the context of the downstream
build, as if the trigger existed immediately after the FROM instruction in
the downstream Dockerfile.
You can register any build instruction as a trigger. A trigger is useful if
you are defining an image to use as a base for building other images. For
example, if you are defining an application build environment or a daemon that
is customized with a user-specific configuration.
Consider an image intended as a reusable python application builder. It must
add application source code to a particular directory, and might need a build
script 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.com Dockerfile
documentation. *Feb 2015, updated by Brian Goff (cpuguy83@gmail.com) for readability