Support exited/dangling _container_ cleanup
The cleanup service should clean up exited / dead containers as well as images and caches. I.e. the equivalent of something like this:
docker ps -a -q -f status=exited -f status=dead | xargs docker rm -v {}
(which also removes volumes, cf. issue #10)
I don't know enough about docker to say if the docker system prune
command (or its equivalent API) mentioned in #10 would take care of this.