image could not be accessed on a registry to record its digest

问题说明：

在管理节点执行docker stack xxx 方式运行服务，报如题错误。

问题原因：

docker swarm运行需要一个镜像仓库才行，所有节点都去这个仓库统一镜像。
来看下官方的解释：

Because a swarm consists of multiple Docker Engines, a registry is required to distribute images to all of them. You can use the Docker Hubor maintain your own.