Currently we're unable to configure the `Docker URL` through JCasC.

The code is missing `setters` for the attribute, and thus we're not even able to do it with the "compatibility"-part of JCasC.

This is the class in question: https://github.com/jenkinsci/docker-build-step-plugin/blob/master/src/main/java/org/jenkinsci/plugins/dockerbuildstep/DockerBuilder.java