title: Visual Studio Team Service - Build Docker image in one phase and use it in another phase
body: <p>I'm creating a build flow in the Visual Studio Team Service portal where I want to achieve the following flow.</p>
  
  <p>Agent phase 1 (1 agent)</p>
  
  <ul>
  <li>Build a docker image from the source</li>
  <li>(maybe push to Azure Container Registry if that is needed to be able to use it in Phase 2?)</li>
  </ul>
  
  <p>Agent phase 2 (n agents)</p>
  
  <ul>
  <li>Build a docker image from the source starting from the docker image built in Phase 1</li>
  <li>Push it to the Azure Container registry</li>
  </ul>
  
  <p>I'm not sure about how I should reference the Docker container built in Phase 1 in my Dockerfile for Phase 2. My guess is that I might need to push it to ACR to fetch it in Phase 2?</p>