Another unfortunate disk space hog I noticed was /var/lib/docker/aufs with a huge 13.4gb, so combined these two directories are using over 30gb of disk space =\
I found this thread https://meta.discourse.org/t/docker-diff-very-big-how-to-clean/12696/5
which also mentions an update and rebuild, so it would be great to figure out how to take care of this error and rebuild the app!