How to transfer a file to fly as a secret

Basically, I have a firebase-service account that requires a JSON file to include when we initialize firebase-admin on the server, but that file should not be committed to remote repositories, so it is in my .gitngore but I want to take that file to fly while deploying can someone give suggestions on how to take a file that is ignored by git but sent that to fly in production

Base64 encode the string and store that as the secret’s value, then decode it to a file on startup before running your app?

There’s a long discussion here about how to store secrets, including files like this: How are you managing cert files with Fly?