Be safe against empty docker_root_dir (issue 1989)

[jsmeix]

• Type: Bug Fix

• Impact: High
  High impact only when docker is used.

• Reference to related issue (URL):
  Ubuntu 14.04 mkbackup does not backup anything #1989 (comment)

• How was this pull request tested?
  I do not use docker and I get exactly the same
  var/lib/rear/layout/disklayout.conf as before
  so I assume there are no regressions.

• Brief description of the changes in this pull request:
  In usr/share/rear/layout/save/GNU/Linux/230_filesystem_layout.sh
  ensure docker_root_dir is not empty because otherwise any
  mountpoint string matches "^" which would skip all mountpoints

[gdha]

@jsmeix do me a favor and replace docker info with timeout 5s docker info - why? I have seen too many cripple docker installation the last couple of months.

[jsmeix]

@gdha
I did even more:
Now it kills 'docker info' with SIGTERM after 5 seconds
plus with SIGKILL after additional 2 seconds.
I can change it as you like.

[jsmeix]

@gdha
only FYI:

On my old SLES10 system there is no timeout command
(at least not by default) and it is not in the 'coreutils' RPM on SLES10.

Since SLES11 there is by default a timeout command that is
provided by the 'coreutils' RPM which is installed by default.

Accordingly since this pull request the command in the line

docker_root_dir=$( timeout -k 2s 5s docker info | grep 'Docker Root Dir' | awk '{print $4}' )

will fail so that docker_root_dir gets empty but I assume that
nobody uses Docker on SLES10 so that in practice this should
not add another regression on SLES10 because the above line
is only run

if service docker status

and that even works on SLES10:

# service docker status && echo Y || echo N
service: no such service docker
N