Ensure license-builder stage only happens when the dockerfile is changed

Signed-off-by: NRenaud Gaubert <rgaubert@nvidia.com>

.gitlab-ci.yml

@@ -12,8 +12,8 @@ license-builder:
   image: docker:stable
   stage: build
   only:
-#    changes:
-#     - license/Dockerfile
+    changes:
+      - license/Dockerfile
   script:
     - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
     - docker build -t $LICENSE_TAG ./license