title: docker build failed. unable to find image with digest
body: <p>trying to build an image an error occurs</p>
  
  <pre><code>Step 16/37 : FROM $IMAGE_PHP as base-backend
   ---&gt; 5fc947f8be08
  Step 17/37 : ARG UID
  unable to find image "sha256:bafb075f25add74a30374cd88b7ab89e39f2931d6e22e64db6fa75d9bf7ee57e"
  </code></pre>
  
  <p>All required images are pulled. IMAGE_PHP has a different digest.
  I don’t understand why docker is looking for an image after the ARG directive.</p>