Try dockerhub again

There were protected variables involved...

.gitlab-ci.yml

@@ -84,6 +84,7 @@ push_dockerhub_registry:
     - docker tag $CI_REGISTRY_IMAGE:latest $DOCKERHUB_REGISTRY_IMAGE:latest
     - docker push $DOCKERHUB_REGISTRY_IMAGE:$CI_COMMIT_TAG
     - docker push $DOCKERHUB_REGISTRY_IMAGE:latest
+  # TODO: Use rules https://docs.gitlab.com/ee/ci/yaml/README.html#rules
   only:
     refs:
 #      - master