Add a cadvisor instance
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 !49 (closed)
Test Plan
http://localhost:5080/cadvisor shows some nice gauges and stuff. http://localhost:5080/prometheus/targets gets a new cadvisor target.
Migrated from D1006 (view on Phabricator)