Exclude btrfs docker submount from being backed up

The backup script fails on our Gitlab machine as it tries to remove a
snapshot of the docker submount, which we do not want to backup anyway.

btrfs subvolume snapshot -r /var/lib/docker/btrfs /var/lib/docker/btrfs/backup-snapshot
ERROR: Not a Btrfs subvolume: Invalid argument

roles/borg_client/templates/borg-backup.sh.j2

@@ -23,7 +23,8 @@ delete_snapshot() {
 }
 
 if is_btrfs "$src"; then
-    list_of_btrfs_submounts=$(findmnt -Rl -o target,fstype,options / | grep btrfs | grep -v "subvol=\/[[:alnum:]]" | cut -d ' ' -f1)
+    # List all btrfs submounts we want to backup, e.g. homedir.archlinux.org with "/ /home"
+    list_of_btrfs_submounts=$(findmnt -Rl -o target,fstype,options / | grep btrfs | grep -v docker | grep -v "subvol=\/[[:alnum:]]" | cut -d ' ' -f1)
 
     if [[ -d "$backup_mountdir" ]]; then
         if [[ $(findmnt -M "$backup_mountdir") ]]; then