code: func DockerContainerHelper(u *url.URL) (*connhelper.ConnectionHelper, error) {
  	container := u.Host
  	return &connhelper.ConnectionHelper{
  		ContextDialer: func(ctx context.Context, addr string) (net.Conn, error) {
  			return commandconn.New(ctx, "docker", "exec", "-i", container, "buildctl", "dial-stdio")
  		},
  	}, nil
  }
docs: // DockerContainerHelper returns helper for connecting to Docker container.
  // docker-container://<container> URL requires BuildKit v0.5.0 or later in the container.