Automate push to registry

[dnephin]

It would be good to have the docker hub/registry updated automatically when a new docker release goes out.

I'm thinking:

• a jenkins job that runs update.sh daily and triggers https://hub.docker.com/r/gliderlabs/dockerhub-tag/ if there was a new commit.
• docker hub automated build

[dnephin]

If you agree, I can work on putting something together

[dnephin]

Actually, automated builds might be an issue because of docker/hub-feedback#289, unless we publish to the docker org on the hub (if I understand it correctly).