Update docker-compose.yaml

[MikePadge]

Howdy, I commented out the rest of the mariadb declaration, and added a comment, since the container does not need to start if postgres is the active default DB for the example.

I also added a volume declaration for postgres to add some default persistence. Might save some new users from running a compose down and wiping their data.

These are just opinions, feel free to accept or not. ¯_(ツ)_/¯ Thanks for this project, it's pretty awesome.