nginx 不会在启动时启动，但手动启动正常。日志说的是
nginx: [emerg] host not found in upstream "{some hostname}" in /etc/nginx/conf.d/some_site.conf
nginx: configuration file /etc/nginx/nginx.conf test failed
Failed to start A high performance web server and a reverse proxy server.
nginx.service: Unit entered failed state.
nginx.service: Failed with result 'exit-code'.
似乎在启动时 dns 还没有准备好，nginx 正在尝试查找我作为反向代理使用的主机名。