Build the docker image only when there're changes

.gitlab-ci.yml

@@ -22,4 +22,11 @@ build:
       - docker push $IMAGE
       - docker tag $IMAGE $RELEASE_IMAGE
       - docker push $RELEASE_IMAGE
+   only:
+     changes:
+      - Dockerfile
+      - pygnunetrest
+      - setup.cfg
+      - setup.py
+      - tests