Add avahi process, log squid3 to stdout.

Resolves #1 and #2.

This is on the hub, too, if you want to try it from there: https://registry.hub.docker.com/u/mikepurvis/squid-deb-proxy/
The `squid-deb-proxy` default is 40GB— you were scaling it back to 8GB here. In any case, I agree that an env var for this would be best.