Docker 支持以下的 64 位 CentOS 版本：
CentOS 7
CentOS 8
更高版本...
如果之前已经安装过Docker在主机上，可以卸载旧版本：
$ sudo yum remove docker \
docker-client \
docker-client-latest \
docker-common \
docker-latest \
docker-latest-logrotate \
docker-logrotate \
docker-engine
在新主机上安装，或者重新安装Docker的客户端和服务端程序。
一键安装Docker的使用国内 daocloud 一键安装命令：
curl -sSL https://get.daocloud.io/docker | sh
测试docker是否成功。
docker -v 是返回docker客户端的版本号。
[root@localhost ~]# docker -v
Docker version 20.10.2, build 2291f61
运行docker的测试hello-world 镜像：
[root@localhost ~]# docker run hello-world
docker: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?.
See 'docker run --help'.
上面错误是因为Docker的服务程序未启动，用systemctl 启动它；
[root@localhost ~]# systemctl start docker
启动服务程序成功后再运行测试：
[root@localhost ~]# docker run hello-world
Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
0e03bdcc26d7: Pull complete
Digest: sha256:1a523af650137b8accdaed439c17d684df61ee4d74feac151b5b337bd29e7eec
Status: Downloaded newer image for hello-world:latest
Hello from Docker!
This message shows that your installation appears to be working correctly.
To generate this message, Docker took the following steps:
1. The Docker client contacted the Docker daemon.
2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
(amd64)
3. The Docker daemon created a new container from that image which runs the
executable that produces the output you are currently reading.
4. The Docker daemon streamed that output to the Docker client, which sent it
to your terminal.
To try something more ambitious, you can run an Ubuntu container with:
$ docker run -it ubuntu bash
Share images, automate workflows, and more with a free Docker ID:
https://hub.docker.com/
For more examples and ideas, visit:
https://docs.docker.com/get-started/
[root@localhost ~]#
上面过程全部成功，表示Docker的客户端和服务端全部安装成功。
