cozystack preset: custom registry mirrors via values

[lexfrei]

Problem

The preset hardcodes a single mirror (docker.io → https://mirror.gcr.io) in both the legacy registries.mirrors block and the multidoc RegistryMirrorConfig. Operators mirroring additional registries (e.g. ghcr.io), or pointing at a private pull-through cache with a custom TLS/insecure setting, must fork the preset.

Proposal

Expose the mirrors via values, driving both the legacy registries block and the RegistryMirrorConfig documents; keep the current gcr mirror as the default:

registryMirrors:
  docker.io: { endpoints: [https://registry.example.com/v2/docker.io] }
  ghcr.io:   { endpoints: [https://registry.example.com/v2/ghcr.io], insecureSkipVerify: true }