How to hide private key in Git?

Question

git

Hello.
There is a small project that lies on heroku. I want to put it on github, but there is a file with a private key for writing to the firebase database.
So. How to do this so that the key is not shared? To push to heroku with a key, and to github without a key, only the code.

Answer 1

https://devcenter.heroku.com/articles/config-vars#...
there should be no secrets in the repository