Heroku gives you the possibility to use a custom buildpack using the BUILDPACK_URL
env variable.
That worked fine with git@github.com:user/repo.git
urls until someone had trouble fetching the buildpack.
Until now I assumed heroku managed to fetch buildpacks on its own but it seems it uses my heroku ssh key to do this.
And it just appears to work because I registered the same ssh key at github. Is it the case ?