The Run syntax to run in the shell at image build time.

See the Command or Entry sections for changing what a container executes.

Run

RUN docs reference.

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.

Two forms:

  • Shell form. 
      RUN command
  • Exec form. 
      RUN ["executable", "param1", "param2"]

e.g.

RUN foo bar

RUN /bin/bash -c 'source $HOME/.bashrc; \
echo $HOME'

Note that the exec form is parsed as a JSON array, which means that you must use double-quotes (“) around words not single-quotes (‘).

RUN ["boo", "bar"]

RUN ["/bin/bash", "-c", "echo Hello"]

# You must use the subshell in exec form to evaluate variables.
RUN [ "sh", "-c", "echo $HOME" ]

Command

See CMD doc reference.

The main purpose of a CMD is to provide defaults for an executing container.

There can only be one CMD instruction in a Dockerfile - only last is used.

The CMD instruction has three forms:

  • Exec form - preferred. 
      CMD ["executable", "param1", "param2"] # (exec form, this is the preferred form)
  • Shell form: 
      CMD command param1 param2

Arguments

ARG doc reference.

Set a build-time arg.

$ docker build --build-arg <varname>=<value>

ARG <name>[=<default value>]

Example:

FROM busybox
ARG user1
ARG buildno
# ...

Using ARG and FROM together, based on the docs.

ARG  CODE_VERSION=latest
FROM base:${CODE_VERSION}
CMD  /code/run-app

FROM extras:${CODE_VERSION}
CMD  /code/run-extras

Entrypoint

An ENTRYPOINT allows you to configure a container that will run as an executable.

  • Exec form - preferred. 
      ENTRYPOINT ["executable", "param1", "param2"]
  • Shell form. 
      ENTRYPOINT command param1 param2

Set default parameters for ENTRYPOINT:

CMD ["param1", "param2"]