Cannot set dockerUrl with CasC

Having installed  CasC and docker-build-step plugins, there is no (obvious way) to set the dockerUrl setting via CasC. The only setting that is shown in the CasC documentation is the `unclassified.dockerBuilder` step. Trying to set `unclassified.dockerBuilder.dockerUrl` results in error. I have tried other combinations as well, but I have not succeeded. If this is possible, it should be documented. If it is no supported, it would be great if support was added.