Only push docker image on master

Author: felixoi

.github/workflows/build.yml

@@ -71,7 +71,7 @@ jobs:
   publish-docker:
     runs-on: ubuntu-latest
     needs: build
-    if: startsWith(github.ref, 'refs/heads/master')
+    if: ${{ github.ref == 'refs/heads/master' }}
     steps:
     - name: Docker Metadata
       id: docker_meta