move Docker manager to docker package

internal/machine/docker/manager_darwin.go

@@ -0,0 +1,14 @@
+//go:build darwin
+
+package docker
+
+import (
+	"context"
+	"fmt"
+	"net/netip"
+)
+
+// EnsureUncloudNetwork is a stub for darwin.
+func (d *Manager) EnsureUncloudNetwork(ctx context.Context, subnet netip.Prefix) error {
+	return fmt.Errorf("not supported on darwin")
+}