$ docker pull php:7.1-fpm-alpine
Error response from daemon: Get https://registry-1.docker.io/v2/: net/http: request canceled while waiting for connection (Client.Timeout exceeded while awaiting headers)
见到这段烦人的超时提示，就知道是网络问题了，解决办法是添加国内镜像
https://docker.mirrors.ustc.edu.cn
https://hub-mirror.c.163.com
http://f1361db2.m.daocloud.io
镜像可以添加多个
设置步骤:
Docker for Windows/Mac
Setting/Preference -> Daemon -> Registry mirrors