The main point was to add some data to prometheus to make sure it
worked; as it turns out, cadvisor is the standard way to add docker/container
metrics to prometheus these days.

It's a bit (ok, _very_) leaky (see the volumes it needs us to mount) so I'm
happy to keep that out, I just figured I'd push it for reference.

Depends on D1004

http://localhost:5080/cadvisor shows some nice gauges and stuff.
http://localhost:5080/prometheus/targets gets a new cadvisor target.