As rsnapshotrsnapshot uses rsyncrsync and passes the exclude rules to it, you can specifically include directories/files by using a '+''+' (include) rule.
The first rule that applies is acted on, so simply add a line
+ /var/lib/docker/volumes/ ABOVE your other rule.
From the rsync documentation:
Filter Rules
...
As the list of files/directories to transfer is built, rsync checks each name to be transferred against the list of include/exclude patterns in turn, and the first matching pattern is acted on: if it is an exclude pattern, then that file is skipped; if it is an include pattern then that filename is not skipped; if no matching pattern is found, then the filename is not skipped.
