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/docker.md

@@ -0,0 +1,42 @@
+---
+title: Docker
+id: docker
+date: 2018-04-12
+full_link: /zh/docs/reference/kubectl/docker-cli-to-kubectl/
+short_description: >
+  Docker 是一种可以提供操作系统级别虚拟化（也称作容器）的软件技术。
+
+aka: 
+tags:
+- fundamental
+---
+
+<!--
+---
+title: Docker
+id: docker
+date: 2018-04-12
+full_link: https://docs.docker.com/engine/
+short_description: >
+  Docker is a software technology providing operating-system-level virtualization also known as containers.
+
+aka: 
+tags:
+- fundamental
+---
+-->
+
+<!--
+Docker (specifically, Docker Engine) is a software technology providing operating-system-level virtualization also known as {{< glossary_tooltip text="containers" term_id="container" >}}.
+-->
+
+ Docker（这里特指 Docker 引擎） 是一种可以提供操作系统级别虚拟化（也称作{{< glossary_tooltip text="容器" term_id="container" >}}）的软件技术。
+
+<!--more--> 
+
+<!--
+Docker uses the resource isolation features of the Linux kernel such as cgroups and kernel namespaces, and a union-capable file system such as OverlayFS and others to allow independent "containers" to run within a single Linux instance, avoiding the overhead of starting and maintaining virtual machines (VMs).
+-->
+
+Docker 使用了 Linux 内核中的资源隔离特性（如 cgroup 和内核命名空间）以及支持联合文件系统（如 OverlayFS 和其他），
+允许多个相互独立的“容器”一起运行在同一 Linux 实例上，从而避免启动和维护虚拟机（VMs）的开销。