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.

README.md

@@ -1,3 +1,11 @@
 # terraform-artifactapi
 
-Terraform configuration for managing ArtifactAPI remote and virtual repositories
+Terraform configuration for managing ArtifactAPI remote, local, and virtual repositories.
+
+Repositories are declared as YAML files under `config/<kind>/<name>.yaml`, where the
+filename is the repository name. `make plan` / `make apply` (via Terragrunt) reconcile
+them against the ArtifactAPI instance.
+
+Kinds: `remote_*` (caching proxies of an upstream), `local_*` (real registries hosted
+directly — `local_docker`, `local_pypi`, `local_rpm`, `local_terraform`), and `virtual`
+(merged views over members of one package type).