title: Error when pushing docker image to registry in Jenkinsfile
body: <p>I'm using a Jenkinsfile to try to push a docker image to my local docker registry in the Jenkins build pipeline plugin.  I understand from reading the docs that you should be able to push an image with an optional tagname: </p>
  
  <p><code>Image.push([tagname])</code></p>
  
  <p>I've got the following code at the moment</p>
  
  <pre><code>  stage('Initialise environment') {
     checkout scm
     db = docker.build('oracle', 'docker/oracle').run("-p 49160:22 -p 49161:1521")
     wlp = docker.build('liberty', 'docker/liberty').run("-p 9080:9080 --link=${db.id}:oracle")
    }
    stage('Push image to registry') {
        docker.withRegistry('https://localhost:5000') {
            db.push()
            wlp.push()
        }
       }
  </code></pre>
  
  <p>It's failing at the first push() with the following error:</p>
  
  <blockquote>
    <p>Failure: groovy.lang.MissingMethodException: No signature of method:
    static org.jenkinsci.plugins.docker.workflow.Docker.push() is
    applicable for argument types: () values: [] Possible solutions:
    use([Ljava.lang.Object;), use(java.util.List, groovy.lang.Closure),
    use(java.lang.Class, groovy.lang.Closure), dump(),
    is(java.lang.Object), each(groovy.lang.Closure)</p>
  </blockquote>
  
  <p>Any ideas? </p>