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/resource_local_docker_test.go

@@ -0,0 +1,125 @@
+package provider
+
+import (
+	"context"
+	"testing"
+
+	"github.com/hashicorp/terraform-plugin-framework/resource"
+	"github.com/hashicorp/terraform-plugin-framework/types"
+)
+
+func TestLocalDockerModelToAPI(t *testing.T) {
+	model := localDockerResourceModel{
+		Name:        types.StringValue("docker-internal"),
+		Description: types.StringValue("Internal container registry"),
+	}
+
+	api := localDockerModelToAPI(model)
+
+	if api.Name != "docker-internal" {
+		t.Errorf("Name: expected docker-internal, got %s", api.Name)
+	}
+	if api.PackageType != "docker" {
+		t.Errorf("PackageType: expected docker, got %s", api.PackageType)
+	}
+	if api.RepoType != "local" {
+		t.Errorf("RepoType: expected local, got %s", api.RepoType)
+	}
+	if api.Description != "Internal container registry" {
+		t.Errorf("Description: expected 'Internal container registry', got %s", api.Description)
+	}
+}
+
+func TestLocalDockerModelToAPI_EmptyDescription(t *testing.T) {
+	model := localDockerResourceModel{
+		Name:        types.StringValue("docker-empty"),
+		Description: types.StringValue(""),
+	}
+
+	api := localDockerModelToAPI(model)
+
+	if api.Name != "docker-empty" {
+		t.Errorf("Name: expected docker-empty, got %s", api.Name)
+	}
+	if api.Description != "" {
+		t.Errorf("Description: expected empty string, got %s", api.Description)
+	}
+	if api.PackageType != "docker" {
+		t.Errorf("PackageType: expected docker, got %s", api.PackageType)
+	}
+	if api.RepoType != "local" {
+		t.Errorf("RepoType: expected local, got %s", api.RepoType)
+	}
+}
+
+func TestLocalDockerAPIToModel(t *testing.T) {
+	api := remoteAPI{
+		Name:        "docker-internal",
+		PackageType: "docker",
+		RepoType:    "local",
+		Description: "Internal container registry",
+		ManagedBy:   "terraform",
+	}
+
+	model := localDockerAPIToModel(api)
+
+	if model.Name.ValueString() != "docker-internal" {
+		t.Errorf("Name: expected docker-internal, got %s", model.Name.ValueString())
+	}
+	if model.Description.ValueString() != "Internal container registry" {
+		t.Errorf("Description: expected 'Internal container registry', got %s", model.Description.ValueString())
+	}
+}
+
+func TestLocalDockerRoundTrip(t *testing.T) {
+	original := localDockerResourceModel{
+		Name:        types.StringValue("roundtrip-docker"),
+		Description: types.StringValue("Round trip test"),
+	}
+
+	api := localDockerModelToAPI(original)
+	result := localDockerAPIToModel(api)
+
+	if result.Name.ValueString() != original.Name.ValueString() {
+		t.Errorf("Name: expected %s, got %s", original.Name.ValueString(), result.Name.ValueString())
+	}
+	if result.Description.ValueString() != original.Description.ValueString() {
+		t.Errorf("Description: expected %s, got %s", original.Description.ValueString(), result.Description.ValueString())
+	}
+}
+
+func TestLocalDockerResource_Metadata(t *testing.T) {
+	r := NewLocalDockerResource()
+	req := resource.MetadataRequest{ProviderTypeName: "artifactapi"}
+	var resp resource.MetadataResponse
+	r.Metadata(context.Background(), req, &resp)
+	if resp.TypeName != "artifactapi_local_docker" {
+		t.Errorf("expected artifactapi_local_docker, got %s", resp.TypeName)
+	}
+}
+
+func TestLocalDockerResource_Schema(t *testing.T) {
+	r := NewLocalDockerResource()
+	req := resource.SchemaRequest{}
+	var resp resource.SchemaResponse
+	r.Schema(context.Background(), req, &resp)
+
+	expectedAttrs := []string{"name", "description"}
+	for _, attr := range expectedAttrs {
+		if _, ok := resp.Schema.Attributes[attr]; !ok {
+			t.Errorf("missing expected attribute: %s", attr)
+		}
+	}
+
+	if len(resp.Schema.Attributes) != len(expectedAttrs) {
+		t.Errorf("expected %d attributes, got %d", len(expectedAttrs), len(resp.Schema.Attributes))
+	}
+}
+
+func TestNewLocalDockerResource_Type(t *testing.T) {
+	r := NewLocalDockerResource()
+	_, ok := r.(*localDockerResource)
+	if !ok {
+		t.Error("expected *localDockerResource")
+	}
+}