It might not be a very useful answer for your imme...

# server

[mp]

It might not be a very useful answer for your immediate question, but I just don’t use Heroku. Buildpacks have been a pain on every single project I’ve ever participated in that used Heroku. IMO it’s a fundamentally wrong-headed approach: deploy artifacts that you (or a process under your control) has built, not source that some other thing then has to figure out how to assemble into something runnable.