It’s been moved to RubyChina mirror. I documented them here and there in Chinese.
You have more than one way to fix this, either by mirrors or by proxy. If you have a reliable proxy, you don’t even bother whether a mirror is perfectly stable to you. Besides, RubyChina mirror is rather stable comparing to the launch. Issues · ruby-china/rubygems-mirror · GitHub
Docker can also employ a proxy as described in the documentation.
If you are using Capistrano, you have more choices since you are a senior developer.