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.

config/config.hcl

@@ -57,6 +57,11 @@ locals {
       trimsuffix(basename(file_path), ".yaml") => content
       if startswith(file_path, "local_rpm/")
     }
+    local_docker = {
+      for file_path, content in local.all_configs :
+      trimsuffix(basename(file_path), ".yaml") => content
+      if startswith(file_path, "local_docker/")
+    }
     virtual = {
       for file_path, content in local.all_configs :
       trimsuffix(basename(file_path), ".yaml") => content

config/local_docker/docker-internal.yaml

@@ -0,0 +1 @@
+description: "Internal container image registry"