Postfix relay host in docker container

Is there a way to setup SASL authentication for postfix relay host in a docker container? I found on git, the, which includes a reference to HOOKROOT=/docker-entrypoint.d. However when I look at my running container, I don’t see the same contents.

I believe that this functionality would allow me to setup SASL. Is the docker build just behind the git repo? Is what’s in git for a different build of checkmk?