I couldn't find a nginx-buildpack that would only serve static files. The most recently updated one requires an app  server to be running to communicate with nginx via sockets, but that's obviously not what we want. I tinkered with a custom buildpack, but unfortunately ran out of time. Also, updated the docs to include a warning.

Ironically, I found deploying to Digitalocean with Vagrant+Chef-solo way easier than messing around with Heroku buildpacks.