Package applications that "are not under my control" (i.e. where I am not the maintainer)?

[kastl-ars]

Dear all,

just getting started with Buildpacks, so this might be a FAQ. But as I have not found it answered, maybe because I am missing the proper search terms...

How do I properly build an image for an application, where I am not the maintainer? This means I cannot add files (Procfile etc.) to the application directory?

Would I add those files "on-the-fly" during the build pipeline, before starting the build with buildpacks?

Kind Regards,
Johannes

[kaminagakur4]

Keep the upstream checkout untouched. Almost everything you'd put in that directory can come from outside instead.

Put your project.toml anywhere and point pack at it with -d. In pack v0.40.9 that flag is declared at build.go#L272; without it, pack just looks for <app>/project.toml. It carries the builder, the buildpack group, build env and include/exclude:

[io.buildpacks]
exclude = [ "secrets/" ]
builder = "paketobuildpacks/builder-jammy-base"

[[io.buildpacks.group]]
uri = "./my-buildpack"

[[io.buildpacks.build.env]]
name = "MY_MSG"
value = "hello"

pack build my-app \
  --path /checkouts/upstream-app \
  --descriptor /my-config/project.toml \
  --default-process my-cli

I tested this with the app directory read-only. One detail: "exclude" matches paths under --path, but a relative buildpack uri resolves from the descriptor's own folder, where pack points ProjectDescriptorBaseDir (build.go#L202). So ./my-buildpack sits beside your project.toml, not beside the app.

The real gap is processes. The 0.2 schema has no process table (v02/project.go#L10-L19), so --default-process only picks among types the buildpacks already produce. If you need one nobody declares, add a small buildpack of your own through that uri and let it write launch.toml. That beats copying a Procfile in mid-pipeline.

References: pack v0.40.9 and the project descriptor spec.