Differences to official PostgreSQL image

[rvansa]

There are several differences to the official image. While some of these could be actually improvements as a user who wants to migrate from the Docker Hub-hosted image this is an annoyance:

• envvars for credentials: POSTGRESQL_* vs. POSTGRES_*
  • the created role is a superuser; this image uses postgres role for the admin ops but the username is non-configurable
• user id 26 vs 999 (PGDATA directory ownership needs to be changed).
• non-configurable PGDATA (and some other args)
• init scripts: official image supports .sql files, not just .sh

[pkubatrh]

We are welcome to PRs for getting the images closer to upstream container images, but not a priority for the team right now. This would need to be ideally non-breaking for existing instances, but not sure if it can be achieved.