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.
