There should be an option for the Docker Pipeline configuration for Declarative Pipelines to point to a Dockerfile - if that option is given, then we should build that Dockerfile and run in the resulting image.
Hrm. Got a frustrating point here - we don't do checkout scm until after we've got our node/docker container/etc, but we need to do that checkout scmbefore we can do docker.build(...). Gotta think about this.