Check for docker sockets named after both _hostname or _fqdn

Check for docker sockets named after both _hostname or _fqdn

While hostname -f will always return an hostname including its
domain part and -s without the domain part, the behavior when
no arguments are given can include or not include the domain part
depending on how the system is configured; the socket name might
not match the instance name then.

(cherry picked from commit bdcc52b96dc1f9c99ce490117170f644623d4846)
Signed-off-by: Sébastien Han <seb@redhat.com>