Create a dummy interface to intergate with systemd-resolved

This is required due to two things:

1. The docker0 interfaces is only "up" when there are containers on the
   default network.
2. systemd-resolved only takes into account interfaces which are up.

If containers were only running in non-default networks (for example
using docker-compose) the systemd-resolved DNS configuration would not
be used since the docker0 interface is down.

To work around this a dummy interface is created which is always up.