I knew repo2docker’s build packs were based on the build packs I know from heroku, but I hadn’t connected Heroku and CNB together. I am off to do some reading now 
Slightly OT: maybe for future decisions in repo2docker we should look at how the CNB work and handle things. Then copy as much as possible from them. This is the “the less code we have to maintain the better” part of me seeing an opportunity to retire repo2docker 
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