* Fix "fatal role postgres doesn't exist"
Fix a frequent error with recent postgres docker images:
`FATAL: role "postgres" does not exist`
* Use $$VAR so it's expanded by the shell, not docker
Invidious gained support to read its configuration from an env var
instead of config file in e3c10d779d.
Unfortunately, Docker doesn't allow newline characters in env var
values (see [0]) which means we can only provide a proper YAML config
by using the inlined configuration in docker-compose.yml which,
unfortunately, is tracked by Git. Once support for multiline env var
values has been added to Docker, we should migrate and read the config
from a .env file instead (which is not tracked by Git).
[0]: https://github.com/docker/compose/issues/3527
