However, as I noted above, this model breaks for disconnected environments, in which neither the staging container nor the runtime container have internet connectivity.
Heroku's ruby buildpack code runs bundler, Cloud Foundry's buildpack is a soft fork of Heroku's, so either you vendor your dependencies before sending it to Cloud Foundry for staging, or you get a failed staging step when the code in the staging container can't dial out to a remote repo.