A Heroku buildpack for setting the ssh private key as part of the application build. It's meant to be used as part of a setup using multiple buildpacks, so other buildpacks can authenticate with hosts using ssh keys, for instance to install dependencies from private git repositories.
Add the buildpack to your Heroku app either using the CLI or the Heroku dashboard. This buildpack needs to run before any other buildpacks make SSH calls (such as the heroku/nodejs buildpack performs an npm install):
$ heroku buildpacks:set --index 1 https://github.com/joshnesbitt/heroku-buildpack-git-rewrite.git
$ heroku buildpacks:add heroku/nodejs