Docker Hub官方Nginx镜像文档
https://hub.docker.com/_/nginx/
跟着文档说明进行配置，有些问题不知道什么意思，
原文引用：

using environment variables in nginx configuration

Out-of-the-box, Nginx doesn't support using environment variables inside most configuration blocks. But envsubst may be used as a workaround if you need to generate your nginx configuration dynamically before nginx starts.

Here is an example using docker-compose.yml:

image: nginx
  volumes:
   - ./mysite.template:/etc/nginx/conf.d/mysite.template
  ports:
   - "8080:80"
  environment:
   - NGINX_HOST=foobar.com
   - NGINX_PORT=80
  command: 
        /bin/bash -c "envsubst < /etc/nginx/conf.d/mysite.template > /etc/nginx/conf.d/default.conf    
        &&    
        nginx -g 'daemon off;'"

The mysite.template file may then contain variable references like this :

listen ${NGINX_PORT};

问题：
在代码部分最后一行，也就是command：那一行，
1、envsubst作用是什么？
2、daemon off;作用是什么？
3、command：命令本身表示什么意思？什么时候执行后面的命令？