从docker pull开始，看 docker image 的存储过程

# docker pull ubuntu

Using default tag: latest

latest: Pulling from library/ubuntu

5ba4f30e5bea: Pull complete

6874f9870f5f: Pull complete

4c876570bd7d: Pull complete

10fb34ebccea: Pull complete

Digest: sha256:f1b592e2de671105255a0c0b7b2f71a92b829403e8fc845e3482667ecc301780

Status: Downloaded newer image for ubuntu:latest

# docker images

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE

ubuntu              latest              12543ced0f6f        2 weeks ago         122.4 MB

其中image名字是ubuntu，image的id是12543ced0f6f，在docker中，几乎所有的ID都是通过UUID或者sha256等方式计算出来的