use ci-templates to pre-bake a docker-image

This splits the pipeline in two, using helpers from the ci-templates repo.

The first step pre-builds an appropriate docker-image, and tags it.

The second step uses the pre-built image instead of re-downloading python2 every time.

The benefit is that we cache the image in the docker-registry, so we don't end up re-downloading this for every build.

Closes: #1 (closed)