build docker image: tag the jobs to use the gstreamer runners

docker builds can be big, and other shared runner have smaller
storage space which we should avoid filling if can be avoided.
The gst tagged runners are a better fit for such builds since
their disk storage is about 1-2 terabytes.

.gitlab-ci.yml

@@ -28,6 +28,7 @@ test manifest:
   image: 'registry.fedoraproject.org/fedora:31'
   extends:
     - '.global_ci_policy'
+  tags: ['gstreamer']
   rules:
     - when: 'manual'
       allow_failure: true