How do I generate a PDF through the gradle DocBookTask

buildscript {
  repositories {
    mavenCentral()
    maven { url "http://maven.restlet.org" }
  }

  dependencies {
    classpath group: 'org.docbook', name: 'docbook-xslt2', version: '2.2.0'
    classpath group: 'com.xmlcalabash', name: 'xmlcalabash1-gradle', version: '1.3.2'
    classpath group: 'org.xmlresolver', name: 'xmlresolver', version: '0.13.1'
    // Add this line:
    classpath group: 'org.docbook', name: 'docbook-schemas', version: '5.1-1'
  }
}

import org.docbook.DocBookTask
import com.xmlcalabash.XMLCalabashTask

task myDocument(type: DocBookTask) {
  // And tell the pipeline to validate with the schema
  option("schema", "https://docbook.org/xml/5.1/rng/docbook.rng")
  input("source", "document.xml")
  output("result", "output.html")
}