Add ability to specify image cache repositories in regopts

[vrga]

Description

Basically, i'm hosting docker-registries running in cache mode in my local network for use with multiple upstream registries (docker.io, ghcr.io, quay.io specifically)
I'd like to configure diun to target my local caches rather than the upstreams.

current configuration is like this example, but obviously, this does not work:

  regopts:
    - name:     'registry-cache-docker.example.com'
      selector: 'docker.io'
    - name:     'registry-cache-ghcr.example.com'
      selector: 'ghcr.io'
    - name:     'registry-cache-quay.example.com'
      selector: 'quay.io'

I'm not a 100% sure if this is just a mistake on my reading of the docs, but basically, the idea would be to override the docker.io part of docker.io/chrislusf/seaweedfs:3.85 for example and redirect the check to my cache.

If this is actually already possible, could a documentation example be provided please?
Apologies if its a nonsensical request 😅

[vrga]

Upon reading a bit of the code and docs, ran across diun.hub_link label https://crazymax.dev/diun/providers/docker/#docker-labels
it seems i can do what i want by specifying the diun.hub_link label on all the containers that i have rolling via the caches, i guess that makes what i want doable, but it would be much nicer if i could specify this "globally" for diun 🤔