feat: manage local docker registries

ArtifactAPI now serves local docker repos as real container registries and the
provider exposes an artifactapi_local_docker resource, but this config had no
way to declare one.

- Add a local_docker variable, module resource, config loader wiring, and
  terragrunt input, mirroring the other local_* kinds.
- Declare a docker-internal registry.
- Expand the README to cover the local_* and virtual kinds.

Depends on a terraform-provider-artifactapi release exposing
artifactapi_local_docker (and a matching .terraform.lock.hcl bump) before apply.

modules/artifactapi/variables.tf

@@ -120,6 +120,14 @@ variable "local_terraform" {
   default = {}
 }
 
+variable "local_docker" {
+  description = "Map of local Docker repositories (real container registries)"
+  type = map(object({
+    description = optional(string, "")
+  }))
+  default = {}
+}
+
 variable "virtual" {
   description = "Map of virtual repositories"
   type = map(object({