title: trigger jenkins build on tag creation with multibranch pipeline
body: <p>I've got a stage in <a href="https://github.com/ryandawsonuk/activiti-build/blob/5d60df0b0e1410f901c64313d90e7aa03054c1df/Jenkinsfile#L79" rel="noreferrer">my Jenkinsfile</a> for building from a tag matching a tag name filter:</p>
  
  <pre><code>      stage('Build Release from Tag') {
          when {
            tag '*RELEASE'
          }
  </code></pre>
  
  <p>In order to get tags to be discovered I've had to add tag discovery and a match for the tag name regex and to my pipeline under <code>Branch sources &gt; GitHub</code> :</p>
  
  <p><a href="https://i.stack.imgur.com/Qncyv.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Qncyv.png" alt="enter image description here"></a></p>
  
  <p><em>The pipeline does discover tags that I push which match the filter. And if I click on one to run a build for it then it does run the build stage that matches the tag conditional. But I was expecting, <a href="https://jenkins.io/blog/2018/05/16/pipelines-with-git-tags/" rel="noreferrer">based on my interpretation of a blogpost</a>, that the build would start automatically as build for branches do.</em> </p>
  
  <p>In the scan repository log I see output such as:</p>
  
  <pre><code>    Checking tag TEST0.1.3.RELEASE
        ‘Jenkinsfile’ found
      Met criteria
  Changes detected: TEST0.1.3.RELEASE (null → 4aea4ec43c1daf8290ea438ce0bf1a14a6afbc46)
  No automatic builds for TEST0.1.3.RELEASE
  </code></pre>
  
  <p>I also see output in the <code>repository events</code> log such as <code>Received Push event for tag TEST.0.1.8.RELEASE in repository ryandawsonuk/activiti-build CREATED event</code> so Jenkins is aware of tags being created. (Which I guess I know already because it discovers them and shows them in the UI, it just doesn't build them unless I manually trigger the build.)</p>
  
  <p>I should say I am using Jenkins-X. As far as I can see this is a Jenkins question and doesn't relate to anything specific to Jenkins-X but I could be overlooking something.</p>
  
  <p>If the pipeline should trigger automatically on tag creation (it seems like Jenkins supports that <a href="https://issues.jenkins-ci.org/browse/JENKINS-47496" rel="noreferrer">but I'm not 100% sure</a>) then I'm wondering what additional configuration I could add to trigger the build automatically? </p>