• DockerServer simply creates dockershim grpc server

// DockerServer is the grpc server of dockershim.
type DockerServer struct {
	// endpoint is the endpoint to serve on.
	endpoint string
	// service is the docker service which implements runtime and image services.
	service DockerService
	// server is the grpc server.
	server *grpc.Server
}
...
// Start starts the dockershim grpc server.
func (s *DockerServer) Start() error {
	glog.V(2).Infof("Start dockershim grpc server")
	l, err := util.CreateListener(s.endpoint)
	if err != nil {
		return fmt.Errorf("failed to listen on %q: %v", s.endpoint, err)
	}
	// Create the grpc server and register runtime and image services.
	s.server = grpc.NewServer()
	runtimeapi.RegisterRuntimeServiceServer(s.server, s.service)
	runtimeapi.RegisterImageServiceServer(s.server, s.service)
	go func() {
		// Use interrupt handler to make sure the server to be stopped properly.
		h := interrupt.New(nil, s.Stop)
		err := h.Run(func() error { return s.server.Serve(l) })
		if err != nil {
			glog.Errorf("Failed to serve connections: %v", err)
		}
	}()
	return nil
}

• DockerService is the interface implement CRI remote service server

// DockerService is the interface implement CRI remote service server.
type DockerService interface {
	runtimeapi.RuntimeServiceServer
	runtimeapi.ImageServiceServer
}

// **dockerService uses dockershim service to implement DockerService**.

BTW, are you sure you will use in the future? From the latest (5 days ago) news: Kubernetes is Moving on From Dockershim: Commitments and Next Steps:

• Kubernetes is removing dockershim in the upcoming v1.24 release.
• If you use Docker Engine as a container runtime for your Kubernetes cluster, get ready to migrate in 1.24
• Full removal is targeted in Kubernetes 1.24, in April 2022.
• We'll support Kubernetes version 1.23, which includes dockershim, for another year in the Kubernetes project.