Should Dockerfiles specify an entry point when using docker compose?-CodePudding

I am migrating some web-apps to be managed via docker compose

It seems the docker-compose.yaml has a section for the container entry-point.

However, my individual docker files have an ENTRYPOINT themselves... should I remove this from the Dockerfiles? Does the entry-point in docker-compose override the Docker one?

CodePudding user response：

You usually shouldn't specify entrypoint: or command: in a Compose file. Prefer specifying these in a Dockerfile. The one big exception is if you have a container that can do multiple things (for example, it can be both a Web server and a queue worker, with the same code) and you need to tell it with a command: to do not-the-default thing.

I'd suggest a typical setup like:

# docker-compose.yml
version: '3.8'
services:
  app:
    build: .
    # with neither entrypoint: nor command:

# Dockerfile
FROM ...
WORKDIR /app
COPY ...
RUN ...

# ENTRYPOINT ["./entrypoint-wrapper.sh"]
CMD ["./my_app"]

Compose entrypoint: overrides the Dockerfile ENTRYPOINT and resets the CMD. Compose command: overrides the Dockerfile CMD.

In the Dockerfile both ENTRYPOINT and CMD are optional. If your base image already includes a correct command setup (nginx, php:fpm) then you can safely skip both.

It's otherwise somewhat a matter of style whether to use CMD or ENTRYPOINT in your Dockerfile. I prefer CMD for two reasons: it's easier to replace in a docker run ... image-name alternate command invocation, and there's a pattern of using ENTRYPOINT as a wrapper script to do first-time setup and then launch the CMD with exec "$@". If you have a JSON-array-syntax ENTRYPOINT then you can pass additional command-line arguments to it as docker run ... image-name --option. Both setups are commonplace.

The thing you shouldn't do is put an interpreter in ENTRYPOINT and a script name in CMD. I only ever see this in Python, but ENTRYPOINT ["python3"] is wrong. On the one hand this is hard to override in the same way ENTRYPOINT is in general, and on the other neither normal command override format works (you still have to repeat the script name if you want to run the same script with different options).