安装docker出现的问题
GPG key retrieval failed: [Errno 12] Timeout on https://download.docker.com/linux/centos/gpg: (28, ‘Operation timed out after 30001 milliseconds with 0 out of 0 bytes received‘)
原因：
由于国内访问不到docker官方镜像的缘故
解决：
执行命令
sudo yum-config-manager --add-repo http://mirrors.aliyun.com/docker-ce/linux/centos/docker-ce.repo
执行后结果：

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

GPG key retrieval failed: [Errno 12] Timeout on https://download.docker.com/linux/centos/gpg: (28, 'Operation timed out after 30001 milliseconds with 0 out of 0 bytes received')