Update build-push-image.yml

Author: Ckster

.github/workflows/build-push-image.yml

@@ -10,6 +10,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v2
+        with:
+          ref: upgrade-to-3.9
 
       - name: Login to DockerHub
         uses: docker/login-action@v1