Docker images have a tag named latest which doesn’t work as you expect.
Latest is just a tag with a special name.
“Latest” simply means “the last build/tag that ran without a specific tag/version specified”.
Just version your tags. Every time.
docker image有一個tag叫做latest，latest通過最近一次沒有指定版本的build或tag來生成
下面實驗來看：
$ docker tag imagename localhost:5000/imagename
$ docker push localhost:5000/imagename
這樣可以在repository上創建一個版本為latest的imagename，下一次更新操作如下：
1）先停止使用舊的image的container並刪除，然后刪除舊的image
$ docker stop $container_id
$ docker rm $container_id
$ docker rmi localhost:5000/imagename
2）同上
$ docker tag imagename localhost:5000/imagename
$ docker push localhost:5000/imagename
這種方式不會保留歷史版本；
更好的做法是：
1）版本1
$ docker tag imagename localhost:5000/imagename:1
$ docker push localhost:5000/imagename:1
$ docker tag imagename localhost:5000/imagename
$ docker push localhost:5000/imagename
這時latest=1
2）版本2
$ docker tag imagename localhost:5000/imagename:2
$ docker push localhost:5000/imagename:2$ docker stop $container_id
$ docker rm $container_id
$ docker rmi localhost:5000/imagename
$ docker tag imagename localhost:5000/imagename
$ docker push localhost:5000/imagename
這時latest=2
參考：
https://medium.com/@mccode/the-misunderstood-docker-tag-latest-af3babfd6375
https://stackoverflow.com/questions/39352186/update-a-docker-image-in-registry