Add public-image tag to opt into scanning (firebase#8642)

Author: joehan

scripts/publish/firebase-docker-image/cloudbuild.yaml

@@ -13,7 +13,7 @@ steps:
     entrypoint: "sh"
     args:
       - "-c"
-      - "docker build -t us-docker.pkg.dev/$PROJECT_ID/us/firebase:$(cat /workspace/version_number.txt) -t us-docker.pkg.dev/$PROJECT_ID/us/firebase:latest -f ./Dockerfile ."
+      - "docker build -t us-docker.pkg.dev/$PROJECT_ID/us/firebase:$(cat /workspace/version_number.txt) -t us-docker.pkg.dev/$PROJECT_ID/us/firebase:latest -t us-docker.pkg.dev/$PROJECT_ID/us/firebase:public-image-$(cat /workspace/version_number.txt) -f ./Dockerfile ."
 
 images:
   - "us-docker.pkg.dev/$PROJECT_ID/us/firebase"