Docker container should be able to interact with host avahi-daemon

[sylveon]

On systems like TrueNAS, it is not possible to disable the host's avahi-daemon, and it takes over the 5353 port on every interface. Which means that the docker container cannot expose printers to the network.

Currently, attempting to bind-mount /var/run/dbus to the docker container in an attempt to make it use the system avahi-daemon makes the host very unhappy because the container's dbus daemon stomps the host's file.

With CUPS, you can bind-mount /var/run/dbus to a container that runs CUPSD and it happily uses that avahi-daemon to share printers over the network.