Run build on master and tags only

.gitlab-ci.yml

@@ -19,5 +19,5 @@ build:
   script:
     - docker build --pull -t "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG" .
     - docker push "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG"
-  except:
-    - master
+  only:
+    - tags