Use container names to manage pool membership, not images

If you `docker pull` to upgrade an image on a live system, containers that were launched with the previous state of that image lose their name and become a garbled SHA. This keeps the SpawnPool from being able to recognize its own containers after a reboot, and can result in the number of containers doubling and making Docker go haywire.

I'm changing the pool to use predictably-patterned container names to manage pool membership, instead.

This will also lead into the ability to maintain pool membership across tmpnb restarts.