Docker compose version 3 doesn't work as I expected

it's my first time working with docker-compose. I have this file:

db:
  image: postgres
  ports:
    - "3600:5432"
  environment:
    - POSTGRES_HOST=127.0.0.1
    - POSTGRES_DB=db
    - POSTGRES_USER=postgres
    - POSTGRES_PASSWORD=postgres1
api:
  build: .
  ports:
    - "4500:4500"
  links:
    - db

And now I have to translate it to version 3:

version: "3"
services:
  db:
    image: postgres
    ports:
      - "3600:5432"
    environment:
      POSTGRES_HOST: "127.0.0.1"
      POSTGRES_DB: "db"
      POSTGRES_USER: "postgres"
      POSTGRES_PASSWORD: "postgres1"
  api:
    build: .
    ports:
      - "4500:4500"
    links:
      - db

It's a node app with postgres linked, and when I'm reading how to connect to the database, I do it like this:

var fs = require('fs');
var config = JSON.parse(fs.readFileSync('./config/prod.json', 'utf8'));

config.database = {
            host:   (process.env.TRAVELOOCAPI_DB_1_PORT_5432_TCP_ADDR || "localhost"),
            port:   (process.env.TRAVELOOCAPI_DB_1_PORT_5432_TCP_PORT || "5432"),
                db:     (process.env.TRAVELOOCAPI_DB_1_ENV_POSTGRES_DB || 'myapp'),
            dialect:     'postgres',
            user:     (process.env.TRAVELOOCAPI_DB_1_ENV_POSTGRES_USER || 'pg'),
            pass:     (process.env.TRAVELOOCAPI_DB_1_ENV_POSTGRES_PASSWORD || 'password'),
        };

It works with first version but it doesn't with second version. There's nothing in process.env related to postgres.

What am I doing wrong?

Thanks!

via Javier Manzano