Add as a first buildpack in the chain. Set PROJECT_PATH environment variable to point to project root. It will be promoted to slug's root, everything else will be erased. Following buildpack (e.g. nodejs) will finish slug compilation.

Disclaimer: I may change the code without notice, so always pin to specific github version. Provided as is.

1. heroku buildpacks:clear if necessary
2. heroku buildpacks:set https://github.com/timanovsky/subdir-heroku-buildpack
3. heroku buildpacks:add heroku/nodejs or whatever buildpack you need for your application
4. heroku config:set PROJECT_PATH=projects/nodejs/frontend pointing to what you want to be a project root.
5. Deploy your project to Heroku.

The buildpack takes subdirectory you configured, erases everything else, and copies that subdirectory to project root. Then normal Heroku slug compilation proceeds.