Docker: Recent changes broke Postgres setup

Some recent changes have broken the database connection on docker setups when upgrading to 1.3rc1

1. After upgrade the API container tries to connect with postgres on host localhost instead of postgres. Setting DATABASE_HOST=postgres in the .env file fixes this
2. Connection to the server fails with django.db.utils.OperationalError: connection to server at "postgres" (172.19.0.7), port 5432 failed: FATAL: role "funkwhale" does not exist.

Not fully sure how to best address this, but here are my suggestions:

1. Set DATABASE_HOST=postgres in the env.prod.sample file and document the change so users can adjust their existing .env file on upgrade
2. Document how to add the "funkwhale" role to the existing postgres container. Obviously this cannot be done by a migration (due to the connection failing), so I assume this is a manual upgrade step

Correction: Actually it is all of the DATABASE_* variables that need different behaviour. So the default environment file should probably contain:

DATABASE_HOST=postgres
DATABASE_USER=postgres
DATABASE_NAME=postgres

to retain the old behaviour.