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.

environments/artifactapi.k8s.syd1.au.unkin.net/terragrunt.hcl

@@ -27,5 +27,6 @@ inputs = {
   local_terraform = local.config.local_terraform
   local_pypi      = local.config.local_pypi
   local_rpm       = local.config.local_rpm
+  local_docker    = local.config.local_docker
   virtual         = local.config.virtual
 }