Fix local-up-karmada.sh is not working.

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

hack/util.sh

@@ -749,7 +749,7 @@ function util::set_mirror_registry_for_china_mainland() {
     "cluster/images/buildx.Dockerfile"
   )
   for dockerfile in "${dockerfile_list[@]}"; do
-    grep 'mirrors.ustc.edu.cn' ${repo_root}/${dockerfile} > /dev/null || sed -i'' -e "1a\\
+    grep 'mirrors.ustc.edu.cn' ${repo_root}/${dockerfile} > /dev/null || sed -i'' -e "/FROM alpine:/a\\
 RUN echo -e http://mirrors.ustc.edu.cn/alpine/v3.17/main/ > /etc/apk/repositories" ${repo_root}/${dockerfile}
   done
 }