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.

internal/provider/provider.go

@@ -71,6 +71,7 @@ func (p *ArtifactAPIProvider) Resources(_ context.Context) []func() resource.Res
 		NewLocalTerraformResource,
 		NewLocalPyPIResource,
 		NewLocalRPMResource,
+		NewLocalDockerResource,
 	}
 }