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/
Minor, I would like to keep an increased cache limit. However the proper way would be to allow user to pass params either via `env` or through `start.sh`. But this should be added if someone actually requests it.