ceph-docker-lint: add docker shellcheck

ceph-docker-lint: add docker shellcheck

To run shellcheck with the -x option (Follow 'source' statements even
when the file is not specified as input.  By default, shellcheck will
only follow files specified on the command line (plus /dev/null).  This
option  allows  following any file the script may source.) we need at
least version 0.4.0.
Unfortunately epel only provides a 0.3.X version.

Fortunately, the container image provides the latest version which helps
us running the tests.

Signed-off-by: Sébastien Han <seb@redhat.com>