docker watcher: discover on container name, not image name

When I first wrote the docker watcher, docker containers could only be identified by their image name. Now there's the ability to attach names to containers: http://blog.docker.io/2013/10/docker-0-6-5-links-container-naming-advanced-port-redirects-host-integration/. This is a much more flexible way to discover them, since most of the time your base image is something basic like "ubuntu:12.04" which lots of services could share.

Start by tracing the Docker watcher and its current image-name discovery path. Update discovery to use container names, then verify that services sharing a base image can be distinguished by their container names.

Written by the indexing model from the issue text.