fix(scheduler): announce proper address

Before, the announce container relied on `getent` to retrieve the
hostname. Some hosts do not have `getent hosts deis` properly
routed, which would make the router crash and burn because it would
try to route traffic to `:1234` instead of `10.0.2.14:1234`.
Instead, the announce container now just does some grokking for the
host's IP address, which is the IP address for the docker bridge,
commonly known as docker0. This bridge is private and can only
be seen by containers on the same network, so I'm not sure how
this will work on a 2+ node cluster.

fixes #797