[Feature] Ability to specify custom Docker registry

[teamwolfyta]

Thankfully, all the container images I use exist on Docker hub but in general I think it'd be extremely useful (dare I say required) that this tool supports the ability to specify a registry as some container images aren't uploaded to Docker hub.

Awesome tool by the way!

[berberman]

Thank you for your feature request. Since nix-prefetch-docker already supports custom registry, e.g. nix-prefetch-docker --json quay.io/prometheus/node-exporter v1.9.1 works without issue, I guess it would be sufficient to add an option that sets registry in nvchecker.