libvips for Heroku builder

Hello everyone,

I’m struggling to install some buildpacks that Rails requires for image-processing. Specifically what I’m trying to do is described here: ruby on rails - Could not open library 'vips.42'; Could not open library 'libvips.42.dylib' - Stack Overflow

I’ve configured fly.toml like this:

  builder = "heroku/buildpacks:20"
  buildpacks = ["", "", "heroku/nodejs", "heroku/ruby"]

The problem is I receive this from the builder:

Error failed to fetch an image or build from source: downloading buildpack: extracting from inspecting buildpack blob: failed to get next tar entry: archive/tar: invalid tar header

I guess what I’m confused about is whether I’m using Heroku buildpacks or whether they’re CNB-specific Heroku buildpacks (or are they the same thing?) and how to properly reference them so that the builder can use them.