Refactor CI begin_script and make docker install idempotent

Move the begin_script part into a common section and make the docker install idempotent by first checking if it is installed or not. This makes it possible to run CI with a prebuilt image that has our required dependencies, instead of starting off from a clean debian image like we do now. It seems to save around 2 minutes per run so probably worth doing, though I haven't taken the plunge and switched images just yet - want to build one that is updated over time.