Docker hub 中的service center 镜像说明需要优化

[liweiv]

1. Dockerfile 链接 404 见后面附图
2. 建议在docherhub 镜像说明文字中将传入的环境变量说明一下，方便用户在不看源码的情况下将etcd集群地址传入，便于基于容器部署SC.
3. scfrontend docker镜像建议 同上

附图：

[liweiv]

SC ENV : BACKEND_ADDRESS for etcd server list scfrontend ENV: BACKEND_ADDRESS for sc http API

变量名建议区分一下，部署的时候容器搞混淆。

[asifdxtreme]

@veily Thanks for pointing out this issue, recently there were some changes in the build script so the Dockerfile was moved.

I have updated the link in the docker hub page, and for the issue 2 & 3 we will work on it and provide the fix in coming release, we will be more than happy if you are willing to submit the fix for this :)

[little-cui]

关于变了命名容易被混淆问题已经提PR，请检视