Bundle "zkctl" binary as well in the docker image of zetcd

[h2002044]

I'm trying to use zetcd for the first time using Docker+Kubernetes. I wanted to debug/try and see whether the zetcd proxy routes the requests to etcd servers.

However, the utility "zkctl" listed in the README.md of the project does not seem to be bundled in the docker image. It would be helpful if it is bundled in the docker image so that we can quickly verify whether the zetcd proxy is routing the requests to the server or not.

[xiang90]

@h2002044

This is a good idea. Would you like to help to make it happen? Thanks.