Adding docker based functional tests to the pipeline

[kessler-frost]

Currently we cannot always test whether the Slurm executor is working as expected functionally due to its need for an actual slurm managed cluster. We can fix that by using a docker based alternative such as https://hub.docker.com/r/turuncu/slurm and run that as part of the functional test suite. This will allow us to do more robust testing and we'll have a reliable way to reproduce if there are any issues.