multibranchPipelineJob has a Properties section with the following fields:

• Docker Label
• Docker registry URL
• Registry credentials

I could not find anything relevant in the Job DSL API viewer on my Jenkins install. I looked at the config.xml and tried something, and it worked! Which means the API viewer is missing that information. Following is working for me and configuring those fields.

multibranchPipelineJob('Foo-Build') {
    properties {
        folderConfig {
            dockerLabel('Foo-Build-Docker')
            registry {
                url('https://harbor/')
                credentialsId(null)
            }
        }
    }
}

Please document folderConfig under properties.