However, starting a new container and running identical commands works.
You will need to explain how the equivalent of postgres.sh is being run in the build variant.
docker run -it postgres sh
To me, these are identical processes, but I may be wrong because docker is docker.
I don't think docker is really the issue here.
Though I am curious that initdb didn't fail during the build since it doesn't seem like either -D or PGDATA have been set and one of those is required. I suppose it ended up using an unexpected location that didn't match up with the -D option to pg_ctl. I'll admit I don't know the exact environment changes being done in the build environment via USER and the runtime environment via gosu. But in any case the claim that the two setups are identical is simply wrong.
It is indeed possible to do what you are trying - to bake in an active database into the docker image, and not rely on a separate mounted volume for the database. But that also isn't really how the existing image is setup to be used and so you will need to figure out the customizations needed to make it do what you want - the documentation and scripts will probably get you close but probably not exactly at your destination.
And as noted, this is not a bug, and even if it was, the core project would not be the team responsible for addressing it. In addition to posting to the docker specific resources if you do want to reach the broader community for help the -general list is what you would want to use.
