fix: auto-detect containerd.sock path and conditionally start unregistry

internal/machine/cluster.go

@@ -215,6 +215,7 @@ func (cc *clusterController) Run(ctx context.Context) error {
 		return nil
 	})
 
+	if cc.unregistry != nil {
 		errGroup.Go(func() error {
 			slog.Info("Starting unregistry server.")
 			if err := cc.unregistry.ListenAndServe(); err != nil {
@@ -222,6 +223,7 @@ func (cc *clusterController) Run(ctx context.Context) error {
 			}
 			return nil
 		})
+	}
 
 	// Wait for the context to be done and stop the network API server.
 	<-ctx.Done()
@@ -230,7 +232,8 @@ func (cc *clusterController) Run(ctx context.Context) error {
 	cc.server.GracefulStop()
 	slog.Info("Network API server stopped.")
 
-	// Stop the unregistry server with a timeout.
+	// Stop the unregistry server with a timeout if it was started.
+	if cc.unregistry != nil {
 		unregTimeout := 30 * time.Second
 		slog.Info("Stopping unregistry server.", "timeout", unregTimeout)
 		unregCtx, cancel := context.WithTimeout(context.Background(), unregTimeout)
@@ -240,13 +243,14 @@ func (cc *clusterController) Run(ctx context.Context) error {
 			return fmt.Errorf("unregistry server forced to shutdown: %w", err)
 		}
 		slog.Info("Unregistry server stopped.")
+	}
 
 	// Wait for all controllers to finish.
 	err = errGroup.Wait()
 
 	// It's safe to stop the Corrosion service after the controllers depending on it and API server are stopped.
 	// Use a new context with a timeout as the current context is already canceled.
-	ctx, cancel = context.WithTimeout(context.Background(), 10*time.Second)
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
 	if corroErr := cc.corroService.Stop(ctx); corroErr != nil {
 		err = errors.Join(err, fmt.Errorf("stop corrosion service: %w", corroErr))

internal/machine/machine.go

@@ -64,6 +64,8 @@ type Config struct {
 
 	// DockerClient manages system and user containers using the local Docker daemon.
 	DockerClient *client.Client
+	// ContainerdSockPath is the path to the containerd.sock used by Docker.
+	ContainerdSockPath string
 
 	// CaddyConfigDir specifies the directory where the machine generates the Caddy reverse proxy configuration file
 	// for routing external traffic to service containers across the internal network. Default is DataDir/caddy.
@@ -94,6 +96,26 @@ func (c *Config) SetDefaults() (*Config, error) {
 		}
 		cfg.DockerClient = cli
 	}
+	if cfg.ContainerdSockPath == "" {
+		// Auto-detect the containerd.sock path used by Docker.
+		paths := []string{
+			"/run/containerd/containerd.sock", // Default path on most Linux distributions.
+			"/run/docker/containerd/containerd.sock",
+			"/var/run/containerd/containerd.sock",
+			"/var/run/docker/containerd/containerd.sock",
+		}
+		for _, path := range paths {
+			if _, err := os.Stat(path); err == nil {
+				cfg.ContainerdSockPath = path
+				slog.Debug("Detected containerd socket used by Docker.", "path", path)
+				break
+			}
+		}
+
+		if cfg.ContainerdSockPath == "" {
+			slog.Warn("Failed to auto-detect containerd socket used by Docker.")
+		}
+	}
 
 	if cfg.CorrosionDir == "" {
 		cfg.CorrosionDir = filepath.Join(cfg.DataDir, "corrosion")
@@ -409,19 +431,23 @@ func (m *Machine) Run(ctx context.Context) error {
 				return fmt.Errorf("create embedded DNS server: %w", err)
 			}
 
+			var unreg *unregistry.Registry
+			if m.config.ContainerdSockPath != "" {
 				// Create an embedded container registry listening on the machine IP address and
 				// using the local Docker (containerd) image store as its backend.
-			unreg, err := unregistry.NewRegistry(unregistry.Config{
+				unreg, err = unregistry.NewRegistry(unregistry.Config{
 					Addr:                net.JoinHostPort(m.IP().String(), strconv.Itoa(constants.UnregistryPort)),
 					ContainerdNamespace: "moby",
-				// TODO: auto-detect the containerd.sock path used by Docker and/or allow configuring it.
+					ContainerdSock:      m.config.ContainerdSockPath,
-				ContainerdSock: "/run/containerd/containerd.sock",
 					LogFormatter:        "text",
 					LogLevel:            "info",
 				})
 				if err != nil {
 					return fmt.Errorf("create embedded registry: %w", err)
 				}
+			} else {
+				slog.Warn("Skipping embedded unregistry setup as the containerd socket path is not configured.")
+			}
 
 			m.mu.Lock()
 			m.clusterCtrl, err = newClusterController(