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)
Mostly because we don't know enough yet about what might change - I want to sync with eric about what he'll do for building kube in brew and then base our env on that.  Don't want to break STI accidentally or incur the extra cost until we settle that.


> On Oct 24, 2014, at 5:00 PM, Ben Parees <notifications@github.com> wrote:
> 
> In hack/buildenv/Dockerfile:
> 
> > @@ -0,0 +1,25 @@
> > +# This file creates a standard build environment for building STI
> @smarterclayton why would we not want to have the multiplatform go builder image available on docker hub and used by both of these projects? Cloning the dockerfile is begging for a maintenance headache.
> 
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