Mongo database in GCP app engine

Question

I'm currently looking into GCP app engine and I was figuring out how I would deploy a very large application with multiple services. I also wanted to use mongodb. GCP docs say that app engine allows dockerfiles and images. What would happen if I used the mongo docker image as a service on app engine? How would it scale it's instances? What will happen to consistency? I'm aware GCP have a third party solution for mongo, but since they allow docker images, what stops me from using it?