I have a App Engine project I am deploying through Gitlab CI to Google Cloud App Engine. The project uses Firebase Admin. To authorize the application a secret file called private-key.json containing the credentials is required.
To handle this file I have stored it in a Gitlab CI file secret and copy it to the working directory during build. In the deployment step I deploy to App Engine with:
gcloud --quiet --project "$PROJECT_ID_PRODUCTION" app deploy app.yaml --version "$CI_COMMIT_SHORT_SHA" --no-promote
When browsing the source on the App Engine version the file is missing. All other files are there.
I was not able to find any documented rule causing this behavior. What am I missing that causes the file not to be deployed?