阻止提交已经存在的tag

https://docs.docker.com/ee/dtr/user/manage-images/prevent-tags-from-being-overwritten/
https://github.com/SUSE/Portus/issues/627
https://github.com/moby/moby/issues/9015#issuecomment-62095426
https://renovatebot.com/blog/docker-mutable-tags
https://medium.com/sroze/why-i-think-we-should-all-use-immutable-docker-images-9f4fdcb5212