Prevent mlocate from indexing /var/lib/docker

The standard updatedb configuration will scan almost all directories on the host and index their contents, except those listed in the PRUNEPATHS configuration setting in /etc/updatedb.conf. As this includes the /var/lib/docker directory, the mlocate.db file can grow to several gigabytes quickly when running just a few dozen containers. Therefore this patch will exclude the /var/lib/docker directory from being indexed as well.