[WIP] Spec out volumes for containers.

Borrowed straight from https://github.com/jupyter/dockerspawner/pull/5, committed via @ssanderson with a bit of changes on our `ContainerConfig`. Still need to make this configurable on tmpnb launch.
This class looks very much like what I'm working on here: https://github.com/quantopian/DockORM/blob/master/dockorm/container.py#L55.  I'd be interested to see if we could find a central place to not duplicate work on this :).