Resolve "调研 SaaS Docker Proxy 修改为国内源"

What does this MR do and why?

为了快速解决国内个别用户反馈 Pull 官方 Docker 镜像慢的问题，所以针对国内 SASS Gitlab， 以 Feature Flag 方式覆盖原来 Hardcode 的 https://registry-1.docker.io 源，改为 使用国内的 Azure 源 http://dockerhub.azk8s.cn。这是一个临时性的快速解决方案。

In order to quickly solve the problem that some Chinese users reported that pull official docker image is slow, so for the Chinese SASS Gitlab, overwrite the original hardcoded https://registry-1.docker.io source with Feature Flag, change it to use the Chinese Azure source http://dockerhub.azk8s.cn. This is a temporary quick fix.

Here is Upstream MR

Screenshots or screen recordings

已跟 @vincent_stchu 在测试环境试验过，流程能跑通，可用。拉取镜像速度确实变快：

We have tested in China SASS server, It works and the speed is faster than before:

@chaomao

Related issue: #2604 (closed)