Aside: Longer-term I'm waiting for there to be more decent "cloud-like" systems for pushing Docker systems around -- call this a scheduler, placement engine, whatever.
At this point the need for the Ansible "docker" module will be greatly reduced, and we'll have something more akin to the nova or ec2 or rackspace module to place containers.
Until then, we have this sucker, which I'm not neccessarily sure I'm content with the idea of it -- it feels like manually managing Xen or KVM instances without cloud software and having a list of which ones you are running on which machines.