Knowledge Base

Dockerfile

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Dockerfile

A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. It is used to build container images.

Format

# Comment
INSTRUCTION arguments

You can use environment variable expansion:

FROM busybox
ENV FOO=/bar
WORKDIR ${FOO}   # WORKDIR /bar
ADD . $FOO       # ADD . /bar
COPY \$FOO /quux # COPY $FOO /quux

If you have a .dockerignore file, you can exclude certain files from the build context. The file is formatted like a .gitignore.

Instructions

FROM

The FROM instruction initializes a new build stage and sets the Base Image for subsequent instructions. As such, a valid Dockerfile must start with a FROM instruction.

FROM [--platform=<platform>] <image>[:<tag>] [AS <name>]

You can also start from nothing:

FROM SCRATCH

RUN

The RUN instruction will execute any commands in a new layer on top of the current image and commit the results. The resulting committed image will be used for the next step in the Dockerfile. The RUN command has two forms:

  • Shell form (the command is run in a shell): RUN <command>
  • Exec form: RUN ["executable", "param1", "param2"]

RUN --mount allows you to create filesystem mounts that the build can access.

# Bind Mount
RUN --mount=type=bind,source=/path,target=/path CMD
 
# TmpFs
RUN --mount=type=tmpfs,target=path,size=20G CMD

CMD

The main purpose of a CMD is to provide defaults for an executing container. These defaults can include an executable, or they can omit the executable, in which case you must specify an ENTRYPOINT instruction as well. There can only be one CMD instruction in a Dockerfile. If you list more than one CMD then only the last CMD will take effect. The CMD instruction has three forms:

  • Exec form (preferred): CMD ["executable","param1","param2"]
  • As default parameters to ENTRYPOINT: CMD ["param1","param2"]
  • Shell form: CMD command param1 param2

LABEL

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"
LABEL com.example.label-with-value="foo"
LABEL version="1.0"
LABEL description="This text illustrates \
that label-values can span multiple lines."

EXPOSE

The EXPOSE instruction informs Docker that the container listens on the specified network ports at runtime. You can specify whether the port listens on TCP or UDP, and the default is TCP if the protocol is not specified. The EXPOSE instruction does not actually publish the port. It functions as a type of documentation between the person who builds the image and the person who runs the container, about which ports are intended to be published.

EXPOSE <port> [<port>/<protocol>...]

ENV

The ENV instruction sets the environment variable <key> to the value <value>.

ENV <key>=<value> ...
ENV MY_NAME="John Doe"
ENV MY_DOG=Rex\ The\ Dog
ENV MY_CAT=fluffy

ADD

The ADD instruction copies new files, directories or remote file URLs from <src> and adds them to the filesystem of the image at the path <dest>.

ADD [--chown=<user>:<group>] [--chmod=<perms>] [--checksum=<checksum>] <src>... <dest>
ADD [--chown=<user>:<group>] [--chmod=<perms>] ["<src>",... "<dest>"]
 
# Checksums and remote files
ADD --checksum=sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d https://mirrors.edge.kernel.org/pub/linux/kernel/Historic/linux-0.01.tar.gz /
 
# Add a git repository
ADD --keep-git-dir=true https://github.com/moby/buildkit.git#v0.10.1 /buildkit

COPY

The COPY instruction copies new files or directories from <src> and adds them to the filesystem of the container at the path <dest>.

COPY [--chown=<user>:<group>] [--chmod=<perms>] <src>... <dest>
COPY [--chown=<user>:<group>] [--chmod=<perms>] ["<src>",... "<dest>"]

You can also copy between multistaged builds:

FROM alpine as builder
RUN touch /test.txt
FROM ubuntu
COPY --from=builder /test.txt /

ENTRYPOINT

An ENTRYPOINT allows you to configure a container that will run as an executable. Command line arguments to docker run <image> will be appended after all elements in an exec form ENTRYPOINT, and will override all elements specified using CMD.

ENTRYPOINT ["executable", "param1", "param2"]

VOLUME

The VOLUME instruction creates a mount point with the specified name and marks it as holding externally mounted volumes from native host or other containers.

VOLUME ["/data"]

USER

The USER instruction sets the user name (or UID) and optionally the user group (or GID) to use as the default user and group for the remainder of the current stage. The specified user is used for RUN instructions and at runtime, runs the relevant ENTRYPOINT and CMD commands.

USER <user>[:<group>]
USER <UID>[:<GID>]

WORKDIR

The WORKDIR instruction sets the working directory for any RUN, CMD, ENTRYPOINT, COPY and ADD instructions that follow it in the Dockerfile. If the WORKDIR doesn’t exist, it will be created even if it’s not used in any subsequent Dockerfile instruction.

WORKDIR /path/to/workdir

HEALTHCHECK

The HEALTHCHECK instruction tells Docker how to test a container to check that it is still working. This can detect cases such as a web server that is stuck in an infinite loop and unable to handle new connections, even though the server process is still running. When a container has a healthcheck specified, it has a health status in addition to its normal status. This status is initially starting. Whenever a health check passes, it becomes healthy (whatever state it was previously in). After a certain number of consecutive failures, it becomes unhealthy.

HEALTHCHECK [OPTIONS] CMD command

The options that can appear before CMD are:

  • --interval=DURATION (default: 30s)
  • --timeout=DURATION (default: 30s)
  • --start-period=DURATION (default: 0s)
  • --start-interval=DURATION (default: 5s)
  • --retries=N (default: 3)
HEALTHCHECK --interval=5m --timeout=3s \
  CMD curl -f http://localhost/ || exit 1

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