Docker build should use external repo.

[DuncanDoyle]

The new setup of the demo init scripts use the external "brms-coolstore-repo" as the rules project repository. The Docker build does however not yet utilise this concept.

The question is whether we should allow the Docker build to use a locally cached repo, or whether we should fetch the Git repo inside the build .... If we choose to first fetch the repo and after that do the build, we should abstract this in an init-docker script IMO.