Simplify development via docker compose

[nachtjasmin]

YAML anchors are nice, but also a bit hard to read. Instead of that,
we're using a dedicated .env file with several values. Also, we provide
a simple no-op configuration that's used as the base.

That sample configuration reads the necessary configuration from the
passed environment variables and therefore makes it easy to adapt for
other container-based deployments, while offering an entry point for
contributions.

[derf]

Thanks \o/

This is neat, though I think we should iron out some kinks before merging it:

• docker-compose.yml sets the "CRON" variable for the worker, but AFAIK there's nothing checking that variable and the worker actually tries to start a second frontend thread instead. One solution would be to check [ -n "$CRON" ] in docker-run.sh
• When starting starting the containers for the first time, postgresql needs some time for initialization, but travelynx and cron try to use it right away and fail. I think docker-compose has some sort of ping / heartbeat / 'is the service really up' that the scripts can use to avoid this situation?

[nachtjasmin]

Ah yes, good catch. Gonna work on those issues as soon as I find my spoons again :3