gitlab-ci.yml: Add openSUSE Leap 15.2 for gitlab CI/CD

Add build-system-opensuse jobs and opensuse-leap.docker dockerfile.
Use openSUSE Leap 15.2 container image in the gitlab-CI.

Signed-off-by: Cho, Yu-Chen <acho@suse.com>
Tested-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20201229085046.8536-1-acho@suse.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>