fix(builder): push locally built slugrunner image to registry

deis/slugrunner:latest and deis/slugrunner in the builder typically differ, resulting in long initial `git push` for apps because we have to push the slugrunner images to the registry. Moving it to the builder fixes this.
You're not piping error output, so your REGISTRY_PORT or REGISTRY_HOST could be "failed to connect to peers" or something.