Deprecated & Gone
It became too easy for someone to accidentally put the read-only key into their public repo. If you're looking at how to do something similar, review https://github.com/timshadel/heroku-buildpack-github-netrc instead.
Heroku buildpack: Private SSH Keys
This buildpack uses a private SSH key exposed to the environment as
SSH_KEY in order to download
later buildpacks and app dependencies via Git+SSH without embedding a password into the url.
$ heroku create --stack cedar --buildpack http://github.com/fs-webdev/heroku-buildpack-ssh-keys.git $ source $ git push heroku master ... -----> Heroku receiving push -----> Fetching custom buildpack -----> Private SSH Keys app detected