This is an issue affecting the Quay proxy cache feature.

When pulling an image from a proxy cache repository, Quay caches only the image layers requested by the client.

So for example, if one image layer is already stored locally, Quay will never request that image layer from the upstream registry and store it in the object storage.

This leads to some issues especially with Clair because it complains about missing image layers when attempting to scan a cached image.

Steps to reproduce the issue:

1. Install Quay 3.10
2. Set up Quay as a proxy cache repository for docker.io
3. Pull 2 images that share some layers(example golang and python images: 

   1. $ podman pull <quay-registry-endpoint>/<cache-org-name>/golang:latest

   2. $ podman pull <quay-registry-endpoint>/<cache-org-name>/python:latest

4. Confirm python image is not scanned