docker/compose中links／external_links的区别
参考：
http://stackoverflow.com/questions/35154441/docker-compose-links-vs-external-links
将两个服务链接 起来，使之可以通信。方法是设置/etc/hosts的域名解析。
external_links
与外部创建（可以是之前调用compose通过其他yml创建的，也可以是手工创建的容器）的容器link起来，使之可以通信。
If you want to link a container inside of the docker-compose.yml to another container that was not included in the same docker-compose.yml or started in a different manner then you can use external_links and you would set the link to the container's name.
