Switch language name 'zh' to 'zh-cn'

This is the first step to rename 'zh' to 'zh-cn'. There are several reasons why we rename the language name.

- The upstream docsy theme changed the language name, leading to many warnings during site build;
  The side-effect is that the i18n strings are no longer working.
- We believe renaming the language is the right thing to do, because this move can make room for other variants of Chinese language, such as 'zh-tw', 'zh-sg' etc.

There would be several follow-ups to this PR, such as fixing the intra-site links, adding redirects etc.

We will lock up changes to zh/zh-cn pages for the moment, until this one gets in.

This PR is based on commit cdad0a7342.

content/zh-cn/docs/reference/glossary/image.md

@@ -0,0 +1,42 @@
+---
+title: 镜像（Image）
+id: image
+date: 2018-04-12
+full_link: 
+short_description: >
+  镜像是保存的容器实例，它打包了应用运行所需的一组软件。
+
+aka: 
+tags:
+- fundamental
+---
+
+<!--
+---
+title: Image
+id: image
+date: 2018-04-12
+full_link: 
+short_description: >
+  Stored instance of a container that holds a set of software needed to run an application.
+
+aka: 
+tags:
+- fundamental
+---
+-->
+
+<!--
+ Stored instance of a {{< glossary_tooltip term_id="container" >}} that holds a set of software needed to run an application.
+-->
+
+镜像是保存的{{< glossary_tooltip text="容器" term_id="container" >}}实例，它打包了应用运行所需的一组软件。
+
+<!--more--> 
+
+<!--
+A way of packaging software that allows it to be stored in a container registry, pulled to a local system, and run as an application. Meta data is included in the image that can indicate what executable to run, who built it, and other information.
+-->
+
+镜像是软件打包的一种方式，可以将镜像存储在容器镜像仓库、拉取到本地系统并作为应用来运行。
+镜像中包含的元数据指明了运行什么可执行程序、是由谁构建的以及其他信息。