Do not use binary packages. [..] Accepts either “:all:” to disable all binary packages, “:none:” to empty the set (notice the colons)
i.e. the command, as it stood, installed binary packages (wheels) only and the explanation was wrong: in the buildpack you're likely to not have all relevant header files that you need for compiling the packages; better to vendor wheels instead.
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As per the documentation of pip
i.e. the command, as it stood, installed binary packages (wheels) only and the explanation was wrong: in the buildpack you're likely to not have all relevant header files that you need for compiling the packages; better to vendor wheels instead.