Summary:

I found "subdir-heroku-buildpack" buildpack helping me to deploy Go api from subdirectories on Heroku

1. Clear existing buildpacks if needed: heroku buildpacks:clear
2. Set the new buildpack: heroku buildpacks:set https://github.com/timanovsky/subdir-heroku-buildpack
3. Add other necessary buildpacks, e.g., heroku/go.
4. Configure the project root with: heroku config:set PROJECT_PATH=backend.
5. Deploy your project to Heroku.

GitHub Repository:
Subdir Heroku Buildpack

Thank you so much you save my life
https://medium.com/@timanovsky/heroku-buildpack-to-support-deployment-from-subdirectory-e743c2c838dd