WIP: Initial take on Review Apps

This is initial changes to build review apps based on GDK:

1. It creates an extended Docker image that is prebuilt: it has all components, it has initial dependencies installed, it has preseeded database,
2. It extends Makefile to allow checking out arbitrary SHA for the gitlab repository.

Currently, the image is manually built and pushed to my docker hub repo. It's to make it easier, now, for testing.