title: Detect if Docker image would change on running build
body: <p>I am building a Docker image using a command line like the following:</p>
  
  <pre><code>docker build -t myimage .
  </code></pre>
  
  <p>Once this command has succeeded, then rerunning it is a no-op as the image specified by the <code>Dockerfile</code> has not changed. Is there a way to detect if the <code>Dockerfile</code> (or one of the build context files) subsequently changes without rerunning this command?</p>