How do I give a container a static ip, or how do I link two Docker containers? – Nginx

I have a container with RabbitMQ, it has to connect to the external IP of the application container, the problem is that this external IP is dynamic and changes every time. Accordingly, I have to manually change the configuration of my RabbitMQ every time. Maybe you can somehow set a static ip for the "App" container so that it does not change or somehow connect these two containers?

version: '3'
services:
#PHP Service
app:
    build:
        context: .
        dockerfile: Dockerfile
    image: digitalocean.com/php
    container_name: app
    restart: unless-stopped
    tty: true
    environment:
        SERVICE_NAME: app
        SERVICE_TAGS: dev
    working_dir: /var/www
    volumes:
        - ./:/var/www
        - ./docker/php/local.ini:/usr/local/etc/php/conf.d/local.ini
    networks:
        - postgres
#Nginx Service
webserver:
    image: nginx:alpine
    container_name: webserver
    restart: unless-stopped
    tty: true
    ports:
        - "80:80"
        - "443:443"
    volumes:
        - ./:/var/www
        - ./docker/nginx/conf.d/:/etc/nginx/conf.d/
    networks:
        - postgres
#Redis
redis:
    image: 'redis:alpine'
    ports:
        - "6379:6379"
#PostgreSQL        
postgres:
    container_name: postgres_container
    image: postgres
    hostname: postgres
    environment:
        POSTGRES_DB: postgres
        POSTGRES_USER: postgres
        POSTGRES_PASSWORD: secret
        PGDATA: /data/postgres
    volumes:
        - postgres:/data/postgres
    ports:
        - "5432:5432"
    networks:
        - postgres
    restart: unless-stopped

pgadmin:
    container_name: pgadmin_container
    image: dpage/pgadmin4
    environment:
        PGADMIN_DEFAULT_EMAIL: ${PGADMIN_DEFAULT_EMAIL:[email protected]}
        PGADMIN_DEFAULT_PASSWORD: ${PGADMIN_DEFAULT_PASSWORD:-admin}
    volumes:
        - pgadmin:/root/.pgadmin
    ports:
        - "${PGADMIN_PORT:-5050}:80"
    networks:
        - postgres
    restart: unless-stopped  
    depends_on: 
        - postgres     
#RabbitMQ
rabbit:
    image: "rabbitmq:3-management"
    hostname: "rabbit"
    environment:
        RABBITMQ_ERLANG_COOKIE: "SWQOKODSQALRPCLNMEQG"
        RABBITMQ_DEFAULT_USER: "rabbitmq"
        RABBITMQ_DEFAULT_PASS: "rabbitmq"
        RABBITMQ_DEFAULT_VHOST: "/"
    ports:
        - "15672:15672"
        - "5672:5672"
    labels:
        NAME: "rabbitmq"    
    networks:
        - postgres     
#ElasticSearch
elasticsearch:
    image: docker.elastic.co/elasticsearch/elasticsearch:5.4.3
    ports:
        - "9200:9200"
        - "9300:9300"
    networks:
        - postgres                      
#Docker Networks
networks:
  postgres:
    driver: bridge
volumes:
    postgres:
        pgadmin: