Docker-compose : Postgresql ECONNREFUSED

Today I try to dockerize an full app ( frontend , API and a database ) so that I could deploy it on a local machine or/and a virtual machine.

When doing :

docker-compose up

I got an "Connect ECONNREFUSED" when I try to start my API that will connect to the database (it uses Sequelize under the hood) with the following env variable :

DATABASE_URL: "postgresql://${POSTGRES_USER:-postgres}:${POSTGRES_PASSWORD:-jy95}@db:5432/${POSTGRES_DB:-sourcecode}"

In docker docs, it is said :

Each container can now look up the hostname web or db and get back the appropriate container’s IP address. For example, web’s application code could connect to the URL postgres://db:5432 and start using the Postgres database.

In PostgreSQL docs, we can see that postgresql:// is also valid :

The URI scheme designator can be either postgresql:// or postgres://. Each of the URI parts is optional.

Here is my full docker-compose.yml file :

version: '3.7'
services:
  frontend:
    image: jy95/sourcecode-front
    restart: always
    depends_on:
      - api
    ports:
      - "80:3000"
      - "443:3000"
    environment:
      API_SERVER: "api:8080"
      CDN_SERVER: "api:8080/files"
  api:
    image: jy95/sourcecode_api
    restart: always
    ports:
      - "8080:3000"
    depends_on:
      - db
    environment:
      DATABASE_URL: "postgresql://${POSTGRES_USER:-postgres}:${POSTGRES_PASSWORD:-jy95}@db:5432/${POSTGRES_DB:-sourcecode}"
      # use a env file if you want to use other values that default ones
      # https://docs.docker.com/compose/environment-variables/#the-env_file-configuration-option
  db:
    image: postgres:12-alpine
    restart: always
    ports:
      - "5432:5432"
    environment:
      # use a env file for that part later
      # https://docs.docker.com/compose/environment-variables/#the-env_file-configuration-option
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: jy95
      POSTGRES_DB: sourcecode
      POSTGRES_PORT: 5432
      # If we want to access more easily to the database (not recommended)
      # PGDATA: /var/lib/postgresql/data/pg_data
    #volumes:
    #  - pg_data:/var/lib/postgresql/data/pg_data

Thanks in advance for the help

PS: The images are already on the Docker Hub