feat: add artifactapi_local_docker resource

The artifactapi server now serves local docker repos as real container
registries, but the provider had no resource to declare one — only remote
docker proxies and local terraform/pypi/rpm repos.

- Add the artifactapi_local_docker resource (package_type=docker,
  repo_type=local), mirroring the other local resources: name + description,
  managed via /api/v2/remotes.
- Register it in the provider and update the resource-count/type tests.
- Add unit tests, an example, and a Local Resources section to the README.

examples/resources/artifactapi_local_docker/main.tf

@@ -0,0 +1,4 @@
+resource "artifactapi_local_docker" "internal" {
+  name        = "docker-internal"
+  description = "Internal container image registry"
+}