问题描述：在使用yum命令安装指定版本docker时报如下错误：

https://download.docker.com/linux/centos/7/x86_64/stable/Packages/docker-ce-18.06.3.ce-3.el7.x86_64.rpm: [Errno 12] Timeout on https://download.docker.com/linux/centos/7/x86_64/stable/Packages/docker-ce-18.06.3.ce-3.el7.x86_64.rpm: (28, 'Operation too slow. Less than 1000 bytes/sec transferred the last 30 seconds')
Trying other mirror.
docker-ce-18.06.3.ce-3.el7.x86 FAILED                                          
https://download.docker.com/linux/centos/7/x86_64/stable/Packages/docker-ce-18.06.3.ce-3.el7.x86_64.rpm: [Errno 12] Timeout on https://download.docker.com/linux/centos/7/x86_64/stable/Packages/docker-ce-18.06.3.ce-3.el7.x86_64.rpm: (28, 'Operation too slow. Less than 1000 bytes/sec transferred the last 30 seconds')
Trying other mirror.
docker-ce-18.06.3.ce-3.el7.x86 FAILED                                          
https://download.docker.com/linux/centos/7/x86_64/stable/Packages/docker-ce-18.06.3.ce-3.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: download.docker.com; Unknown error"
Trying other mirror.


Error downloading packages:
  docker-ce-18.06.3.ce-3.el7.x86_64: [Errno 256] No more mirrors to try.

原因分析：这是由于国内访问不到docker官方镜像的缘故

解决办法：

使用命令：

sudo yum-config-manager --add-repo http://mirrors.aliyun.com/docker-ce/linux/centos/docker-ce.repo

执行结果如下：

[root@node1 etc]# sudo yum-config-manager --add-repo http://mirrors.aliyun.com/docker-ce/linux/centos/docker-ce.repo
Loaded plugins: fastestmirror
adding repo from: http://mirrors.aliyun.com/docker-ce/linux/centos/docker-ce.repo
grabbing file http://mirrors.aliyun.com/docker-ce/linux/centos/docker-ce.repo to /etc/yum.repos.d/docker-ce.repo
repo saved to /etc/yum.repos.d/docker-ce.repo
[root@node1 etc]#

其本质是修改了docker-ce.repo文件，文件路径：/etc/yum.repos.d/docker-ce.repo

修改后：

[docker-ce-stable]
name=Docker CE Stable - $basearch
baseurl=https://mirrors.aliyun.com/docker-ce/linux/centos/7/$basearch/stable
enabled=1
gpgcheck=1
gpgkey=https://mirrors.aliyun.com/docker-ce/linux/centos/gpg

[docker-ce-stable-debuginfo]
name=Docker CE Stable - Debuginfo $basearch
baseurl=https://mirrors.aliyun.com/docker-ce/linux/centos/7/debug-$basearch/stable
enabled=0
gpgcheck=1
gpgkey=https://mirrors.aliyun.com/docker-ce/linux/centos/gpg

......略

执行命令后，在执行下面命令就不会报错了。

sudo yum install docker-ce-18.06.3.ce-3.el7