在很多的docker的说明中，我们都能看到“layer”的身影，那么到底什么是”layer“？
参见http://docker-doc.readthedocs.org/en/latest/terms/layer.html中的说明：
When Docker mounts the rootfs, it starts read-only, as in a traditional Linux boot, but then, instead of changing the file system to read-write mode, it takes advantage of a union mount to add a read-write file system over the read-only file system. In fact there may be multiple read-only file systems stacked on top of each other. We think of each one of these file systems as a layer.
简单翻译如下：
当docker加载rootfs时，和传统的linux系统中一样，它是只读的，但是随后，不是将当前文件系统变成可读写，而是利用union mount添加一个可读写的文件系统在当前的只读系统上。实际上将会有很多的只读文件系统叠加在彼此之上。我们将这里的每一层文件系统称之为"layer"。
参考阅读：
https://docs.docker.com/engine/understanding-docker/#how-does-a-docker-image-work
https://www.ctl.io/developers/blog/post/caching-docker-images/