Only auth and push on master branch (#29995)

.github/workflows/build_runner_image.yml

@@ -25,6 +25,7 @@ on:
   pull_request:
     branches: ['master']
     paths: ['.github/gh-actions-self-hosted-runners/arc/images/**']
+  workflow_dispatch:
 env:
   docker_registry: us-central1-docker.pkg.dev
   docker_repo: apache-beam-testing/beam-github-actions/beam-arc-runner
@@ -41,6 +42,7 @@ jobs:
       with:
         ref: ${{ github.event.pull_request.head.sha }}
     - name: Authenticate on GCP
+      if: github.ref == 'refs/heads/master'
       uses: google-github-actions/setup-gcloud@v0
       with:
         service_account_email: ${{ secrets.GCP_SA_EMAIL }}
@@ -61,7 +63,7 @@ jobs:
           ${{env.docker_registry}}/${{env.docker_repo}}:latest
           ${{env.docker_registry}}/${{env.docker_repo}}:${{ github.sha }}
     - name: Push Docker image
-      if: github.event_name == 'push'
+      if: github.ref == 'refs/heads/master'
       id: docker_build
       uses: docker/build-push-action@v4
       with: