Rear excludes all filesystems if docker is running, backup is empty

Issue

• ReaR backup finishes very quickly and the backup.tar.gz is empty:

  Creating tar archive '/mnt/rear/rear.5udelk5Llkqj/outputfs/server1/backup.tar.gz'
  Preparing archive operationOK
  

• We can't see the tar command completed, for example:

  Creating tar archive '/tmp/rear.1B6hO6CKD381XH1/outputfs/rhel7/backup.tar.gz'
  Archived 824 MiB [avg 6490 KiB/sec] OK
  Archived 824 MiB in 131 seconds [avg 6441 KiB/sec]
  Exiting rear mkbackup (PID 11625) and its descendant processes