docker-compose.yaml

version: '3'

services:
  nginx:
    image: nginx
    ports:
      - 80:80
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf
      - wordpress:/usr/share/nginx/html
  
  wordpress1:
    image: wordpress:5.4.2-php7.2-apache
    restart: always
    environment:
      WORDPRESS_DB_HOST: db
      WORDPRESS_DB_USER: exampleuser
      WORDPRESS_DB_PASSWORD: examplepass
      WORDPRESS_DB_NAME: exampledb
    volumes:
      - ./wordpress:/var/www/html
    
  wordpress2:
    image: wordpress:5.4.2-php7.2-apache
    restart: always
    environment:
      WORDPRESS_DB_HOST: db
      WORDPRESS_DB_USER: exampleuser
      WORDPRESS_DB_PASSWORD: examplepass
      WORDPRESS_DB_NAME: exampledb
    volumes:
      - ./wordpress:/var/www/html

  wordpress3:
    image: wordpress:5.4.2-php7.2-apache
    restart: always
    environment:
      WORDPRESS_DB_HOST: db
      WORDPRESS_DB_USER: exampleuser
      WORDPRESS_DB_PASSWORD: examplepass
      WORDPRESS_DB_NAME: exampledb
    volumes:
      - ./wordpress:/var/www/html

  db:
    image: mysql:5.7
    restart: always
    environment:
      MYSQL_DATABASE: exampledb
      MYSQL_USER: exampleuser
      MYSQL_PASSWORD: examplepass
      MYSQL_RANDOM_ROOT_PASSWORD: '1'
    volumes:
      - db:/var/lib/mysql

volumes:
  wordpress:
  db:

nginx.conf

events { worker_connections 1024; }

http { 
  upstream wordpress { # configura um pool de endereço de servidores 
      server wordpress1;
      server wordpress2;
      server wordpress3;
  }
  
  server { # configura esse servidor
      listen 80 default_server; # escutando por conexões na porta 80
      listen [::]:80 default_server;

      root /usr/share/nginx/html;
      index index.php;

      location / { # repassa todos os requests para um dos endereços do upstream
        proxy_pass  http://wordpress;  # esse endereço aponta para o upstream wordpress
        add_header X-Upstream $upstream_addr; # adiciona o header Host com o valor de um dos endereços configurados no upstream
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header x-forwarded-for $proxy_add_x_forwarded_for;
      }
  }
}