use buildkit in ci

.github/workflows/gcr.yaml

@@ -19,5 +19,7 @@ jobs:
       uses: actions/checkout@v2.3.4
     - name: Build
       run: |
-        docker build . -t gcr.io/porter-dev-273614/porter-prov:latest -f ./docker/Dockerfile
+        BUILDKIT=1 docker build . -t gcr.io/porter-dev-273614/porter-prov:latest -f ./docker/Dockerfile
+    - name: Push
+      run: |
         docker push gcr.io/porter-dev-273614/porter-prov:latest