You'll need to set up a Github PAT in order to connect your Aserto account to GHCR.
Navigate to the "Connections" tab:
Click on the "Add a connection" button:
Github Container Registry (PAT) provider, name the connection "ghcr" and then use your GitHub PAT to complete the form. Finally, click "Add connection"
Once you've added the new registry, you'll be able to select it when creating policy instances.