This script should be executed regularly on all build slaves that
have docker in order to discard unused images/layers. It would
be a good idea to call "fstrim /" afterwards in order to get more
SSD performance. However, the latter requires root access, and hence
cannot be called by the 'osmocom-build' user and thus jenkins.
Maybe we should install it as a cron job or systemd periodic timer job?
Related: OS#3144
Change-Id: I688b952578507a9cc28fe682221b5c7e3a245519