When a container starts, Docker mount a filesystem under /var/lib/docker/devicemmapper/mnt/. Docker creates these on the fly and they are all rather short lived. Diamond collects them ( T177052 ) and they should all be purged from the labs graphite at some point.
The metrics to purge on labs are of the form:
labs
*.*.diskspace._var_lib_docker*
Or:
./*/*/diskspace/_var_lib_docker*