Card devexp_415 - Allow for custom images for builds

See the `customBuild.json` for example (will be replaced with full example, serves just for testing now ;-).

TODO:

- [x] Add tests
- [ ] Add example
Constant good!  Unused constant bad


> On Dec 10, 2014, at 7:55 PM, Ben Parees <notifications@github.com> wrote:
> 
> In pkg/build/controller/strategy/util.go:
> 
> > @@ -7,20 +7,24 @@ import (
> >  	kapi "github.com/GoogleCloudPlatform/kubernetes/pkg/api"
> >  )
> >  
> > +// DockerSocketPath is the default path for the Docker socket inside the builder
> > +// container
> > +const DockerSocketPath = "/var/run/docker.sock"
> now you're just trying to make me look bad... :)
> 
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