Switch to multi-platform release build

Update build-release to match latest state of openshift/origin. It now uses a docker image to create releases for mac(darwin), win, and linux(amd64)
Why do you need your own dockerfile?  sti-builder is going to be an image in Origin.

----- Original Message -----
> > @@ -0,0 +1,25 @@
> > +# This file creates a standard build environment for building STI
> 
> hmm, so the Dockerfile for the image has a WORKDIR that is based on the
> project's name:
> ```
> WORKDIR /go/src/github.com/openshift/origin
> ```
> but I guess we could make it more generic.
> 
> @smarterclayton would you be ok with separating the buildenv image into its
> own repository and pushing it to dockerhub?
> 
> ---
> Reply to this email directly or view it on GitHub:
> https://github.com/openshift/source-to-image/pull/72/files#r19362981