Docker entrypoint injection

Docker containers are configured with ENTRYPOINTs, or commands that are run when the container is initialized. This can be something like the container base image’s /sbin/init or tinyinit. Sometimes however, there is a need to run something before a container’s normal entrypoint. Under Docker Compose this is as easy as injecting a command before the container entrypoint:

    entrypoint: /bin/sh
    command:
      - -c
      - |
        # f.e. cache new libs
        ldconfig

        # anything else you need to do pre-init can go here

        # the original entrypoint/command should go here
        # for example:
        /sbin/init -- /bin/supercoolprogram

I was inspired by @regbo on GitHub and their use of YAML conventions to make injecting scripts easier.