Upgrade Kaniko example in doc/ci/docker/using_kaniko.md

doc/ci/docker/using_kaniko.md

@@ -66,8 +66,8 @@ build:
     - mkdir -p /kaniko/.docker
     - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
     - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
-  only:
-    - tags
+  rules:
+    - if: $CI_COMMIT_TAG
 ```
 
 ## Using a registry with a custom certificate