only run buildx on master branch

.gitlab-ci.yml

@@ -20,6 +20,8 @@ buildx:
     - export DOCKER_BUILDKIT=1
     - git clone git://github.com/docker/buildx ./docker-buildx
     - docker build --platform=local -o . ./docker-buildx
+  only:
+    - master
 
 build_latest:
   stage: build_latest