Hi Sir.
This PoC would be fine considering topics as follows:
1. For public github repos with docker-compose you must consider that they can not deploy in Google App Engine. In this case each service must be delivery as dockerfile and will be one Ap Engine instance (or instances group).
2. For public github repos non-linux you must consider to compile it previously to buid docker image.
3. What kind of enviroment of GAE will be used to deploy the app? Flex or estandard?.
4. Finally, the automatic process (create project, enable API Regin (for each project), API's authentication, gcloud init and sdk scripts for deployment must be implemented "after click in your public web site" using some additional tools or VMs for continous integration.
5. As I see, this interesting project requires more technical specifications (and probably restrictions) to get the goal: Automatically delivery for an app from github repos.
I have more than 5 years in Google Cloud Platform and more than 10 years of IT experience.
Please feel free to contact me for further details.
Thanks