Add as a first buildpack in the chain. Set SWAP_SUBDIR_NAME environment variable to point to subdirectory that will become root. It will be promoted to slug's root, everything else will be moved into subdirectory within new root. Name of subdirectory for moved root can be defined in SWAP_ROOT_NAME optional environment variable, if not specified it will be "root". Following buildpack (e.g. nodejs) will finish slug compilation.

How to use:

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.

How it works

The buildpack moves all slug content into a subfolder(provided in SWAP_ROOT_NAME) and copies content from subfolder(provided in SWAP_SUBDIR_NAME) into slug root. Then normal Heroku slug compilation proceeds.

If your file structure is

folder1
  - file1
  - file2
folder2
  - file3
  - file4
folder3
  - file5
  - file6
 file7
 file8

And heroku environment variables set to

SWAP_SUBDIR_NAME=folder2
SWAP_ROOT_NAME=app

Resulting file structure will be

app
  - folder1
    - file1
    - file2
  - folder2
    - file3
    - file4
  - folder3
    - file5
    - file6
  - file7
  - file8
file3
file4

As you can see, all content was moved into "app" folder and "folder2" content became a root content