minor fixes to CI script

.gitlab-ci.yml

@@ -10,10 +10,13 @@ stages:
 before_script:
   -  docker login -u gitlab-ci-token -p $CI_BUILD_TOKEN $CI_REGISTRY
 
-build:test:
+build:$suffix:
   image: docker:latest
   stage: build
   script:
     - suffix=$(echo $CI_COMMIT_BRANCH | cut -d "-" -f 2)
     - docker build --rm --no-cache -t $RIFT_IMAGE:$suffix --file Dockerfile .
     - docker push $RIFT_IMAGE:$suffix
+  only:
+    changes:
+      - Dockerfile