Use Git + SSH to do authentication

Really love dockerize! Nice work. :-)

We're exclusively relying on SSH to authenticate (agent forwarding) with github to avoid having to keep credentials stored persistently on remote servers. I've added a bunch of code to check for ssh auth first before going down the road of using credentials.

I'm not very good at shell scripting so any pointers are appreciated. I'd love to see git/ssh support merged eventually though.

Please let me know.
Sebastian