Heroku buildpack to let you add an SSH private key to an heroku app so it can access private GitHub repositories during bundle install
The key have to be stored in the application's env:
heroku config:set GITHUB_DEPLOY_KEY="-----BEGIN RSA PRIVATE KEY-----...."Then you have to install this buildpack on your app: Instructions
A great way to test is using Heroku's buildpack test runner. See https://github.com/heroku/heroku-buildpack-testrunner. To set up, run these commands:
heroku create --buildpack https://github.com/heroku/heroku-buildpack-testrunner
git push heroku master