> the difference is between Docker and Google App Engine

Neither the question nor the answer in your link compare Docker and App Engine.

Docker is a containerization platform, whereas App Engine is a PaaS (platform as a service). Their functions are wholly different.

Very, very loosely; Docker is about building and running an individual package of some software. Basically, bundling software and running bundles. PaaS are platforms for developing, running, and managing (often pre-bundled) software.