Improve install of prerequisite software

We used to condition the installation of all prerequisite software applications on the existence of docker. The thinking was that either someone used like debian:buster as a base, in which case none of the prerequisites, docker least of all, would be installed. Then our install would be triggered. Alternatively, the ci-runner-image we build would be used, and it would have all the prerequisites already installed.

However, in some environment, a CI runner image is used that already has docker but is missing some of the other prerequisites. The installation procedure is now slightly rewritten to test for all prerequisites and trigger the install lacking any of them. A second check is performed for docker, as docker requires adding a repo and a few other steps. I'm hesitant about its idempotency and thus wrapped it in this extra check - plus it's probably faster if docker is already installed too.