An image is a lightweight, stand-alone, executable package that includes everything needed to run a piece of software, including the code, a runtime, libraries, environment variables, and config files.
docker提供的上应用层的抽象，虚拟机还抽象了硬件
docker swarm 用于创建docker主机集群,swarm manager用于管理swarm,Swarm Manager 是swarm中唯一一台能够接受命令，认证其他机器作为工作者的机器
docker swarm init
docker stack deploy -c xxx.yml appname
down the app
docker stack rm appname
down the swarm
dock swarm leave --force