docker-compose up启动报错：Unsupported config option for services service: 'nginx'

我的docker-compose.yml文件如下：

root@root:/home/ghost# cat docker-compose.yml 

version: '2'

networks:

  ghost:

services:

  ghost-app:

    build: ghost

    networks:

      - ghost

    depends_on:

      - db

    ports:

      - "2368:2368"

  nginx:

    build: nginx

    networks:

      - ghost

    depends_on:

      - ghost-app

    ports:

      - "80:80"

  db:

    image: "mysql:5.7.15"

    networks:

      - ghost

    environment:

      MYSQL_ROOT_PASSWORD: mysqlroot

      MYSQL_USER: ghost

      MYSQL_PASSWORD: ghost

    volumes:

      - $PWD/data:/var/lib/mysql

    ports:

      - "3306:3306"

目录结构如下：

root@root:/home/ghost# ls

data  docker-compose.yml  ghost  nginx

root@root:/home/ghost# ls nginx/

Dockerfile  nginx.conf

root@root:/home/ghost# cat nginx/nginx.conf 

worker processes 4;

events {worker connections 1024;}

http {

    server {

        listen 80;

        location / {

            proxy pass http://ghost-app:2368;

        }

    }

}

root@root:/home/ghost# cat nginx/Dockerfile 

FROM nginx

COPY nginx.conf /etc/nginx/nginx.conf

EXPOSE 80