从一个正在运行的container提取出docker image，保存到一个tar文件里，方便传输。

docker commit my-container
sha256:84eeb121cf5d95565c3c6e1a4a5d1fcc2cfd2598e549246e65bb40f8b14aaef3

docker images
REPOSITORY              TAG                 IMAGE ID            CREATED             SIZE
<none>                  <none>              84eeb121cf5d        3 seconds ago       64.2MB

docker tag 84eeb121cf5d my-img:18.04

docker save --output my-img.tar my-img:18.04

docker load --input my-img.tar