如果存在旧版本，则先卸载

最好先将镜像导出保存，以免升级后丢失或者无法使用

如有正在运行的容器，先停止

$ docker ps -q | xargs docker stop

关闭docker服务

$ systemctl stop docker.service

$ systemctl stop docker.socket

查看已经安装的docker

$ apt list --installed | grep docker

lxc-docker/unknown,now 1.9.1 amd64 [installed]

lxc-docker-1.9.1/unknown,now 1.9.1 amd64 [installed,automatic]

将其卸载掉

$ apt remove lxc-docker

$ apt remove lxc-docker-1.9.1

安装docker最新版本

确定已经安装curl命令，没有安装，则运行

$ apt install curl

运行命令安装

$ curl -fsSL https://get.docker.com/ | sh

安装完成后会有提醒

If you would like to use Docker as a non-root user, you should now consider

adding your user to the "docker" group with something like:

  sudo usermod -aG docker your-user

Remember that you will have to log out and back in for this to take effect!

WARNING: Adding a user to the "docker" group will grant the ability to run

         containers which can be used to obtain root privileges on the

         docker host.

         Refer to https://docs.docker.com/engine/security/security/#docker-daemon-attack-surface

         for more information

意思是：

当要以非root用户可以直接运行docker时，需要执行 sudo usermod -aG docker runoob 命令，然后重新登陆，否则会报错

重启docker服务

$ systemctl restart docker

查看版本

$ docker version

Client:

 Version:           18.09.5

 API version:       1.39

 Go version:        go1.10.8

 Git commit:        e8ff056dbc

 Built:             Thu Apr 11 04:44:28 2019

 OS/Arch:           linux/amd64

 Experimental:      false

Server: Docker Engine - Community

 Engine:

  Version:          18.09.5

  API version:      1.39 (minimum version 1.12)

  Go version:       go1.10.8

  Git commit:       e8ff056

  Built:            Thu Apr 11 04:10:53 2019

  OS/Arch:          linux/amd64

  Experimental:     false