Make one Docker Compose service know the domain name of another - networking

Question

I'm using Docker Compose to create two containers. One runs an Nginx web server which serves the mydomain.com website, and the second needs to send HTTP requests to the first one (using the mydomain.com domain name).
I don't want to have to check the Nginx container's ip each time I run it and then use docker run --add-host on the second container. My goal is to run docker-compose up and that everything be ready.
I know it's not possible, but what I'm looking for is something in the line of:
# docker-compose.yml
nginx_container:
...
second_container:
extra_hosts:
# This is invalid. extra_hosts only accepts ips.
- "mydomain.com:nginx_container"

Answer 1

You can get a similar result using a configuration like this:
version: "3"
services:
api:
image: node:8.9.3
container_name: foo_api
domainname: api.foo.test
command: 'npm run dev'
links:
- "mongo:mongo.foo.test"
- "redis:redis.foo.test"
volumes:
- .:/app
- /app/node_modules
ports:
- "${PORT}:3000"
- "9229:9229"
depends_on:
- redis
- mongo
networks:
- backend
redis:
image: redis:3
container_name: foo_redis
domainname: redis.foo.test
ports:
- "6379:6379"
networks:
- backend
mongo:
image: mongo:3.6.2
container_name: foo_mongo
domainname: mongo.foo.test
ports:
- "${MONGO_PORT}:27017"
environment:
- MONGO_PORT=${MONGO_PORT}
networks:
- backend
networks:
backend:
driver: bridge