Description of problem:
If docker is running but has no Docker Root Dir defined, all FS will be ignored during rear mkbackup.

Version-Release number of selected component (if applicable):
rear-2.4-9.el7_7.x86_64

How reproducible:
Always

Steps to Reproduce:
1. run docker without Docker Root Dir
2. run rear mkbackup
3.

Actual results:
All FS excluded:


Expected results:
FS other than docker included and backup completes.

Additional info:
This is due to the check in 230_filesystem_layout.sh that tries to avoid docker FS:
---
 # docker specific exclude part
        if is_true $docker_is_running ; then
            # docker daemon/service is running
            docker_root_dir=$( docker info 2>/dev/null | grep 'Docker Root Dir' | awk '{print $4}' )
            # If $docker_root_dir is in the beginning of the $mountpoint string then FS is under docker control
            # and we better exclude from saving the layout,
            # see https://github.com/rear/rear/issues/1749
            Log "$device is mounted below $docker_root_dir (mount point $mountpoint is under docker control), skipping."
            echo "$mountpoint" | grep -q "^$docker_root_dir" && continue
        fi
---
=> It is based on the output of "docker info" and greps the entry 'Docker Root Dir'.

Already fixed upstream:
https://github.com/rear/rear/pull/2021