Nginx load balance with docker

docker-compose.yml

version: "2"

services:
  gate:
    image: nginx:latest
    ports:
      - 80:80
    volumes:
      - ./load-balance.conf:/etc/nginx/conf.d/default.conf
  web1:
    image: nginx:latest
    volumes:
      - ./serve.conf:/etc/nginx/conf.d/default.conf
  web2:
    image: nginx:latest
    volumes:
      - ./serve.conf:/etc/nginx/conf.d/default.conf

load-balance.conf

upstream foo {
  server web1;
  server web2;
}

server {
  listen 80 default;

  location / { 
    proxy_pass http://foo;
  }
}  

serve.conf

server {
  listen 80;
  location / {
    add_header content-type text/html;
    return 200 'server_addr is $server_addr';
  }
}