how to clean up docker overlay directory?

I'm running docker via CoreOS and AWS's ECS. I had a failing image that got restarted many times, and the containers are still around- they filled my drive partition. Specifically, /var/lib/docker/overlay/ contains a large number of files/directories.

I know that docker-cleanup-volumes is a thing, but it cleans the /volumes directory, not the /overlay directory.

docker ps -a shows over 250 start attempts on my bad docker container. They aren't running, though.

Aside from rm -rf /var/lib/docker/overlay/*, how can I/should I clean this up?