Heroku buildpack to let you add an SSH private key to an heroku app's built dyno, and runs an ssh-agent process there, so it can access SSH secured resources, say through something like Signpost.
The key have to be stored in the application's env:
heroku config:set SSH_KEY="-----BEGIN RSA PRIVATE KEY-----...."This buildpack will also look for a config var named GITHUB_DEPLOY_KEY to match https://github.com/Shopify/heroku-buildpack-git-deploy-keys.
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