docker 默认存储的文件在/var/lib/docker
    layered architecture 多层架构
    layer层
    applicable 适用的

    在container layer 是read write
    在container layer层前面的，是read only
    when destroyed the container layer and all of tthe changes are clear.

    the same image layer is shared all container.
    volume 卷
    volume mounting 批量安装