WIP: Add startup script using environment to configure

[jpotisch]

Add startup script and environment file to allow easy creation of time machine docker container with full network discovery.

Marking as draft because:

1. I haven't tested it yet! Initial backup is still running and I don't want to interrupt it to test. But current environment is working great when run with a hardcoded version of this script (e.g. not refactored to use environment variables) so it's probably good or at least very close.
2. Docker compose is likely the right way to do this but I'm not as familiar with the syntax, I saw some posts suggesting I might not be able to set everything up with compose, and, per point 1 above, I can't test until my initial backup completes!

So this won't merge until it's at least tested, and possibly ported to docker-compose.