Add apt buildpack

Merged David Caro requested to merge add_apt_buildpack into main

This adds fagiani/apt buildpack (the one that supports heroku-22 stack).

It will download it from our clone on gitlab when building (instead of going to github directly).

Note that this version of the implementation will block any buildpack that tries to install a package from any other repo or any url, only allowing the system repositories to be used.

Depends on toolforge-builds-api!12

Edited by David Caro

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