docker-compose on cloudbuild / networking issues - networking

Question

Having the wollowing docker-compose file :
version: '3'
services:
postgres:
image: postgres:12.1
environment:
- POSTGRES_PASSWORD:'xyz'
ports:
- '5432:5432'
volumes:
- postgres:/var/lib/postgresql/data
- ./init.sql:/docker-entrypoint-initdb.d/init.sql
redis:
image: 'redis:5-alpine'
command: redis-server
ports:
- '6379:6379'
volumes:
- 'redis:/data'
specs:
image: gcr.io/project_id/container_name:latest
depends_on:
- 'postgres'
- 'redis'
build: .
entrypoint:
- /bin/bash
- -c
- |
...stuff that require database processing...
environment:
- RAILS_ENV=test
- REDIS_URL=redis://redis:6379/0
- DATABASE_URL=postgres://postgres:#postgres:5432/test_db
links:
- postgres
- redis
volumes:
redis:
postgres:
networks:
default:
external:
name: cloudbuild
and the following cloudbuild step (we're using the community cloud builder) :
...
- name: 'gcr.io/app-dmx-sh/docker-compose'
args:
- 'run'
- 'specs'
id: 'specs'
...
we keep hitting the following error :
could not translate host name "postgres" to address: Name or service not known
We've also tried to add the network_mode: cloudbuild for each containers in the compose file, no success.
What should be done to be able have the compose file network work as expected within this cloudbuild environment ?