I have an nginx config file like this:

location /mag {
  proxy_pass http://localhost:9999;
}
location / {
  root /var/www/html/;
  index index.html;
}

And This is docker-compose file I use:

version: '3'

services:
  db:
    image: mysql:8.0
    container_name: db
    restart: always
    hostname: db
    environment:
      MYSQL_ROOT_PASSWORD: rootpass
      MYSQL_DATABASE: wordpress
      MYSQL_USER: wordpress
      MYSQL_PASSWORD: wordpress
    volumes:
      - /home/wordpress/mysql_data:/var/lib/mysql

  wordpress:
    ports:
      - 127.0.0.1:9999:80
    depends_on:
      - db
    image: wordpress:php7.4
    container_name: wordpress
    restart: always
    environment:
      - WORDPRESS_DB_HOST=db:3306
      - WORDPRESS_DB_USER=wordpress
      - WORDPRESS_DB_PASSWORD=wordpress
      - WORDPRESS_DB_NAME=wordpress
    volumes:
      - /home/wordpress/wordpress_data:/var/www/html:rw

When I call http://mywebsite.com/mag, I expect it to redirect to http://mywebsite.com/mag/wp-admin/install.php, but it redirects to http://mywebsite.com/wp-admin/install.php and it returns 404 error.

What is wrong with what? Would you please help me?