镜像的Layer

在docker docs里面有一句话：

We’ve already seen that Docker images are read-only templates from which Docker containers are launched. Each image consists of a series of layers. Docker makes use of union file systems to combine these layers into a single image. Union file systems allow files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system.

对于docker的images，是由一层层的layer组成的，然后通过联合挂载的方式挂载成一个文件系统。

假设你有以下的dockerfile:

FROM ubuntuENV http_proxy 10.144.xx.xx:8080ENTRYPOINT ["/usr/bin/bash"]

首先你选择了基础镜像ubuntu，这个镜像有很多层。可以从/val/lib/docker下面找到该镜像的信息和每一层的信息。

然后ENV是设置了一个环境变量，这句命令同样会产生一个layer，再然后就是需要执行的命令，同样会产生一个layer。如果你更改了某一层的信息，那么从这层之后所有的层都需要重新build。

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