Limit watchtower scope and integrate better with systemd

[rothgar]

fixes #49

By not using oneshot service type and not detaching the docker compose command you can use normal systemd tools (e.g. journalctl) for logs. Docker commands still work too.

[axfelix]

Inclined to merge this, @bnewbold ?

[bnewbold]

I think improvements here would be good, and these look like probably the right direction. I am not super familiar with watchtower, and know process monitoring stuff can be subtle.

I'd want somebody to do an independent pass over this to check the watchtower docs, and to try setting up a PDS this way and confirm sudo systemctl start/stop works as expected.