Adding GitHub related environment variables for source control

[fjcoulon]

Hello, thanks for providing this to facilitate the setup for a retool instance. I would like to request something so we don't have to modify anything from the already provided source code.
As of today there is a series of variables already configured like the LICENSE_KEY, POSTGRES_HOST, etc.
The request is to add these 3 variables GITHUB_APP_PRIVATE_KEY, GITHUB_APP_INSTALLATION_ID and GITHUB_APP_ID, to the task definitions, so we can use them to integrate GitHub.
Not sure if this is something already in place with another variable name but it seems that is not.
If any question please let me know.