I don’t know if this will help you, but I wrote a tutorial on how to setup a local registry on the LAN on a Fedora Server or RHEL-compatible server. https://techne.hyperreal.coffee/tutorials/setup-a-lan-container-registry-with-podman-and-self-signed-certs/

But anyway, it’s unlikely docker.io or quay.io or ghcr.io will go completely offline. If anything they might experience a DDoS, in which case I imagine they have competent devops employees who would ensure they become functional again within a matter of hours.