gitlab-ci: restrict CI to master branch

.gitlab-ci.yml

@@ -7,3 +7,5 @@ build-container:
   script:
     - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
     - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:build 2>&1 | grep -v "Adding "
+  only:
+    - master