Allow appending commands to Dockerfile's CMD/ENTRYPOINT from Docker-Compose and Docker Run

[ehawman]

Description

Please see docker/compose#3140.

Currently, putting anything in entrypoint for docker-compose.yml will result in the Dockerfile entries for both CMD and ENTRYPOINT being discarded.

The community consensus in the referenced issue is that we would like the ability to append commands before/after CMD or ENTRYPOINT inside docker-compose.yml. I believe the cleanest solution would be to expose the original CMD or ENTRYPOINT as variables for docker-compose to use.

entrypoint: ./wait-for-it.sh -t 60 --strict mydb:3306 && sh -c $DOCKERFILE_CMD

I'm not 100% sure if a similar solution would slot nicely into Docker Run, given that different shells may use different variable syntax.

Perhaps some another arg, --entrypoint-behavior="before" (after, overwrite) would add the functionality without breaking existing flows.

Is this the correct project to host this discussion?