Anyway to set the Docker image entrypoint when building the image or some equvalent?

Question

I have a Dockerfile that I use to build the same image but for slightly different purposes. Most of the time I want it to just be an "environment" without a specific entrypoint so that the user just specifies that on the Docker run line:

docker run --rm -it --name ${CONTAINER} ${IMAGE} any_command parameters

But for some applications I want users to download the container and run it without having to set a command.

docker build -t ${IMAGE}:demo (--entrypoint ./demo.sh) <== would be nice to have

Yes, I can have a different Dockerfile for that, or append an entrypoint to the basic Dockerfile during builds, or various other mickey-mouse hacks, but those are all just one more thing that can go wrong, adding complexity, and are workarounds for the essential requirement.

Any ideas? staged builds?

Answer 1

The Dockerfile CMD directive sets the default command. So if your Dockerfile ends with

CMD default_command

then you can run the image in multiple ways

docker run "$IMAGE"
# runs default_command

docker run "$IMAGE" any_command parameters
# runs any_command instead

A container must run a command; you can't "just run a container" with no process in it.

You do not want ENTRYPOINT here since its syntax is noticeably harder to work with at the command line. Your any_command would be passed as arguments to the entrypoint process, rather than replacing the built-in default.