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/versions.tf

@@ -3,7 +3,7 @@ terraform {
   required_providers {
     artifactapi = {
       source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/artifactapi"
-      version = "== 0.1.3"
+      version = "0.1.3"
     }
   }
 }