docker container throws error: php_network_getaddresses: getaddrinfo failed: Name or service not known

[admin]

Please check my docker-compose file:

version: '2'
services:
  db:
    image: mysql:latest
    networks:
      app_net:
        ipv4_address: 172.16.238.10
        ipv6_address: 2001:3984:3989::10
    volumes:
      - "./data/db:/var/lib/mysql"
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: wordpress
      MYSQL_DATABASE: wordpress
      MYSQL_USER: wordpress
      MYSQL_PASSWORD: wordpress
  wordpress:
    image: wordpress
    links:
     - db
    ports:
      - "8000:80"
    environment:
     - WORDPRESS_DB_PASSWORD=wordpress
    networks:
      app_net:
        ipv4_address: 172.16.238.12
        ipv6_address: 2001:3984:3989::10
    volumes:
     - ./code:/code
     - ./html:/var/www/html


networks:
  app_net:
    driver: bridge
    driver_opts:
      com.docker.network.enable_ipv6: "true"
    ipam:
      driver: default
      config:
      - subnet: 172.16.238.0/24
        gateway: 172.16.238.1
      - subnet: 2001:3984:3989::/64
        gateway: 2001:3984:3989::1

Now Two container are running :
<a href="

https://imgur.com/5sqYL

" rel="noreferrer"><img src="

https://imgur.com/5sqYL

" alt="enter image description here"></a>

IP address for the two containers are properly configured. But when i connect to the <a href="http://172.16.238.12:8000/" rel="noreferrer">http://172.16.238.12:8000/</a> (172.16.238.12 - Is the IP Address of the wordpress). It says, "Unable to connect".

When i check logs files of the docker_wordpress_1, it is showing me :

Warning: mysqli::mysqli(): (HY000/2002): php_network_getaddresses: getaddrinfo failed: Name or service not known in - on line 10

MySQL Connection Error: (2002) php_network_getaddresses: getaddrinfo failed: Name or service not known

I didnot understand what went wrong.