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 -- (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 the CMD, the executes in /bin/sh -c: FROM ubuntu CMD echo "This is a test." | wc - If you run 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.

EXPOSE --EXPOSE [...] 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 The ENV instruction sets the environment variable to the value . This value is passed to all future RUN instructions. This is functionally equivalent to prefixing the command with =. 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 =.

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 ... The ADD instruction copies new files, directories or remote file URLs to the filesystem of the container at path .
Mutliple 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). 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 -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.com Dockerfile documentation.

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