title: Why doesn't my newly-created docker have a digest?
body: <p>I have been following the <a href="https://docs.docker.com/engine/tutorials/dockerimages/#/image-digests" rel="noreferrer">Docker tutorial here</a>, and built a test image on my local OSX machine by committing changes to an existing image and tagging it with three different labels:</p>
  
  <pre><code># docker images
  REPOSITORY               TAG                 IMAGE ID            CREATED             SIZE
  adamatan/sinatra         devel               fccb6b4d21b4        8 minutes ago       469.5 MB
  adamatan/sinatra         junk                fccb6b4d21b4        8 minutes ago       469.5 MB
  adamatan/sinatra         latest              fccb6b4d21b4        8 minutes ago       469.5 MB
  </code></pre>
  
  <p>However, none of these images has a digest:</p>
  
  <pre><code># docker images --digests adamatan/sinatra
  REPOSITORY          TAG                 DIGEST              IMAGE ID            CREATED             SIZE
  adamatan/sinatra    devel               &lt;none&gt;              fccb6b4d21b4        9 minutes ago       469.5 MB
  adamatan/sinatra    junk                &lt;none&gt;              fccb6b4d21b4        9 minutes ago       469.5 MB
  adamatan/sinatra    latest              &lt;none&gt;              fccb6b4d21b4        9 minutes ago       469.5 MB
  </code></pre>
  
  <p>Other test images I have created with a Dockerfile do have a digest.</p>
  
  <p><strong>Why do some images have a digest and some don't? Is it related to the way the images were created (Dockerfile or not)?</strong></p>