GCloud Rails application and keeping Redis password in an .env file

I have a Rails 5.2.3 (Ruby 2.6.3) project hosted on Google Cloud.

I'm currently working on setting up Sidekiq and Redis, following this guide from Google's docs.

I've decided to go with the foreman option in the Deploying to App Engine flexible environment section of the docs, and am stuck on figuring out the app.yaml file. Specifically, I'm stuck on how I can supply the Redis URL, Redis password and Rails Secret Key without exposing it in plaintext in my git history.

I can't find info about this anywhere on the GCloud site. The only mention of ENV files I found are these pages of the GCloud docs, but these only make it so that I can specify specific ENV variables to make available to my app on GCloud, which isn't applicable to me in this situation.

Is there any way for me to have the app.yaml file read from my .env, or even better, from my secrets.yaml file?