Update staging.yaml

.github/workflows/staging.yaml

@@ -31,7 +31,7 @@ jobs:
         EOL
     - name: Build
       run: |
-        DOCKER_BUILDKIT=1 docker build . -t gcr.io/porter-dev-273614/porter-prov:latest -f ./docker/Dockerfile
+        DOCKER_BUILDKIT=1 docker build . -t gcr.io/porter-dev-273614/porter:staging -f ./docker/Dockerfile
     - name: Push
       run: |
-        docker push gcr.io/porter-dev-273614/porter-prov:latest
+        docker push gcr.io/porter-dev-273614/porter:staging