Now that the fastpath is dockerized thanks to #935, we need to setup a code pipeline to build and publish the docker image to Dockerhub so that it can be easily retrieved by the fastpath EC2 instance in AWS

See:

• https://github.com/ooni/backend/tree/master/ooniapi#makefile
• https://github.com/ooni/backend/blob/master/ooniapi/services/oonirun/buildspec.yml