image_builder: try to interrupt script-stage

.travis.yml

@@ -22,7 +22,7 @@ git:
 before_script:
   - docker pull ${DOCKER}
 script:
-  - [[ -z TARGET_REF ]]
+  - [[ -z ${TRAVIS_TAG} ]] || exit 1
   - docker run --privileged --rm -v /dev:/dev -v ${BUILD_DIR}:/builder/image -e "TARGET_REPO=${TARGET_REPO}" -e "TARGET_REF=${TARGET_REF}" ${DOCKER}
 # after_failure:
 #   - skip_deploy: true