I'm using Google Kubernetes Engine, Cloud Build, and Image Registry. According to the kubectl docs, I can use external images in Dockerfiles with COPY --from. This would be very useful because when I run gcloud builds submit on my Dockerfile, I'd like to add in images already built on GCR instead of rebuilding everything in one Dockerfile.

I've tried adding lines like COPY --from=quickstart-image:latest /some/path/thing.conf /thing.conf but I always get

pull access denied for quickstart-image, repository does not exist or may require 'docker login'

Is there some authentication step I'm missing? How can I get this to work?