Suggested changes to the docker deployment documentation

[dreamerblue]

Hi, I have successfully deployed Outline but after some testing and tweaking, the following parameters are recommended to be changed in document Docker.
This change was made to help users avoid error FATAL: role "root" does not exist from postgres container (due to pg_isready trying to test with default root user). I'd like to submit a PR to help improve the documentation but can't find the code base of documentation, so I'm using discussion to suggest improvements.

Suggested changes:

postgres:
  # existed configs...
  healthcheck:
    test: ["CMD", "pg_isready -d outline -U user"]
    # ... remaining configs