原有nginx镜像中CMD是，如下图
如果想自定义启动脚本, 最终在启动nginx
以下是自定义脚本内容：特别注意nginx的启动和原有启动保持一致
官方解释
If you add a custom CMD in the Dockerfile, be sure to include -g daemon off;
in the CMD in order for nginx to stay in the foreground,
so that Docker can track the process properly (otherwise your container will stop immediately after starting)!
如果你在Dockerfile中添加了一个自定义CMD，一定要关闭-g守护进程;
为了让nginx留在前台，
这样Docker就可以正确地跟踪进程(否则你的容器会在启动后立即停止)!
