title: In my Dockerfile, how do I use the `COPY --from` syntax to use images from Google Container Registry?
body: <p>I'm using Google Kubernetes Engine, Cloud Build, and Image Registry. <a href="https://docs.docker.com/develop/develop-images/multistage-build/#use-an-external-image-as-a-stage" rel="nofollow noreferrer">According to the kubectl docs</a>, I can use external images in Dockerfiles with <code>COPY --from</code>. This would be very useful because when I run <code>gcloud builds submit</code> on my Dockerfile, I'd like to add in images already built on GCR instead of rebuilding everything in one Dockerfile.</p>
  
  <p>I've tried adding lines like <code>COPY --from=quickstart-image:latest /some/path/thing.conf /thing.conf</code> but I always get</p>
  
  <p><code>pull access denied for quickstart-image, repository does not exist or may require 'docker login'</code></p>
  
  <p>Is there some authentication step I'm missing? How can I get this to work?</p>