We’re in the process of replacing builtins with buildpacks.
You can get started by running flyctl launch in your app directory which will create and configure a fly app then deploy it. Or if you already have a fly app, replace the [build] section in your fly.toml file with this to switch over:
[build]
builder = "paketobuildpacks/builder:base"
buildpacks = ["gcr.io/paketo-buildpacks/nodejs"]
Let me know if you have trouble.