Tweaked MCVE to try to reproduce problems in https://stackoverflow.com/questions/58817846/docker-compose-hostname-to-communicate-between-containers-works-with-postgres-bu

0-docker-compose.yml

---
version: "3.7"

services:

  postgres:
    image: "postgres:11.0-alpine"

  app:
    build: .
    ports:
      - "4000:4000"
    depends_on:
      - postgres
      - nuxt

  nuxt:
    image: node:latest
    ports:
      - "3000:3000"
    command: 'bash -c "while true; do sleep 2000; done"'

1-Dockerfile

# Dummy file to have an app service
FROM debian:buster

CMD while true; do sleep 2000; done

2-Interactive session

# Note: "toto" is the name of my test folder (hence docker-compose project name).

$ docker-compose up -d
Creating network "toto_default" with the default driver
Creating toto_nuxt_1     ... done
Creating toto_postgres_1 ... done
Creating toto_app_1      ... done

$ docker-compose ps
     Name                    Command               State           Ports         
---------------------------------------------------------------------------------
toto_app_1        /bin/sh -c while true; do  ...   Up      0.0.0.0:4000->4000/tcp
toto_nuxt_1       docker-entrypoint.sh bash  ...   Up      0.0.0.0:3000->3000/tcp
toto_postgres_1   docker-entrypoint.sh postgres    Up      5432/tcp

$ docker-compose exec nuxt bash
root@428c01bd1363:/# ping postgres
PING postgres (192.168.80.3) 56(84) bytes of data.
64 bytes from toto_postgres_1.toto_default (192.168.80.3): icmp_seq=1 ttl=64 time=0.356 ms
64 bytes from toto_postgres_1.toto_default (192.168.80.3): icmp_seq=2 ttl=64 time=0.171 ms
^C
--- postgres ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 0.171/0.263/0.356/0.093 ms
root@428c01bd1363:/# ping app
PING app (192.168.80.4) 56(84) bytes of data.
64 bytes from toto_app_1.toto_default (192.168.80.4): icmp_seq=1 ttl=64 time=0.108 ms
64 bytes from toto_app_1.toto_default (192.168.80.4): icmp_seq=2 ttl=64 time=0.088 ms
64 bytes from toto_app_1.toto_default (192.168.80.4): icmp_seq=3 ttl=64 time=0.115 ms
^C
--- app ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2046ms
rtt min/avg/max/mdev = 0.088/0.103/0.115/0.016 ms