Use docker-compose and a well-known Avahi image

[snorp]

The current image tries to embed Avahi itself, which can cause problems if other containers or even the host is already running this. It may be better to simply rely on an existing image and use docker-compose to connect things.

[mrstegeman]

This is the setup we had up until a couple months ago. Not having avahi causes mDNS lookup failures, though.

What would be preferred is an environment variable or something to selectively enable/disable avahi-daemon.