Adding docker-builder queue

[jszwedko]

It might be nice to have an easy way to queue jobs via the docker-builder binary.

E.g. docker-builder queue --repo github.com/foo/bar --ref 123fe21

It could read the environment for the docker builder server name and credentials (as well as allowing them to be specified as flags). You could also default to the current repo and ref (if running from a git repo) if desired.

[rafecolton]

I like it! Happy to review a pull request if you get to it before me 😉

[rafecolton]

Done.