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
