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/main.tf

@@ -110,6 +110,13 @@ resource "artifactapi_local_rpm" "this" {
   description = each.value.description
 }
 
+resource "artifactapi_local_docker" "this" {
+  for_each = var.local_docker
+
+  name        = each.key
+  description = each.value.description
+}
+
 resource "artifactapi_virtual" "this" {
   for_each = var.virtual