add a case for using tag

Signed-off-by: Yi Zha <[email protected]>

Author: yizha1

articles/container-registry/container-registry-tutorial-sign-build-push.md

@@ -191,6 +191,12 @@ The following steps show how to create a self-signed certificate for testing pur
     IMAGE=$REGISTRY/${REPO}@$DIGEST
     ```
 
+    In this tutorial, if the image has already been built and is stored in the registry, the tag serves as an identifier for that image for convenience.
+
+    ```bash
+    IMAGE=$REGISTRY/${REPO}@$TAG
+    ```
+
 3. Get the Key ID of the signing key. A certificate in AKV can have multiple versions, the following command gets the Key Id of the latest version.
 
     ```bash