diff --git a/ubuntu-cn/Dockerfile b/ubuntu-cn/Dockerfile
index 5b8fcf8..65eeb19 100644
--- a/ubuntu-cn/Dockerfile
+++ b/ubuntu-cn/Dockerfile
@@ -1,2 +1,3 @@
 FROM ubuntu:20.04
-RUN sed 's/archive.ubuntu.com/mirrors.aliyun.com/g; s/security.ubuntu.com/mirrors.aliyun.com/g' /etc/apt/sources.list > /etc/apt/sources.list.new && mv /etc/apt/sources.list.new /etc/apt/sources.list && rm -f /etc/apt/apt.conf.d/docker-gzip-indexes
+RUN sed -i 's/archive.ubuntu.com/mirrors.aliyun.com/g; s/security.ubuntu.com/mirrors.aliyun.com/g' /etc/apt/sources.list \
+    && rm -f /etc/apt/apt.conf.d/docker-gzip-indexes
diff --git a/ubuntu-cn/Readme.md b/ubuntu-cn/Readme.md
new file mode 100644
index 0000000..2fab6af
--- /dev/null
+++ b/ubuntu-cn/Readme.md
@@ -0,0 +1,5 @@
+# ubuntu-cn
+
+apt源更换为 [阿里云 Ubuntu镜像](https://developer.aliyun.com/mirror/ubuntu), 加速大陆用户镜像构建.
+
+移除了 [`docker-gzip-indexes`](https://github.com/moby/moby/blob/9a9fc01af8fb5d98b8eec0740716226fadb3735c/contrib/mkimage/debootstrap#L118-L130) 解决部分场景下apt拉取失败的问题.