version: '3'
services:
  web:
    image: odoo:12.0
    depends_on:
      - db
    ports:
      - "8069:8069"
  db:
    image: postgres:10
    environment:
      - POSTGRES_PASSWORD=odoo
      - POSTGRES_USER=odoo
      - POSTGRES_DB=postgres
If the default postgres credentials does not suit you, tweak the environment variables:

version: '2'
services:
  web:
    image: odoo:12.0
    depends_on:
      - mydb
    ports:
      - "8069:8069"
    environment:
    - HOST=mydb
    - USER=odoo
    - PASSWORD=myodoo
  mydb:
    image: postgres:10
    environment:
      - POSTGRES_USER=odoo
      - POSTGRES_PASSWORD=myodoo
      - POSTGRES_DB=postgres
Here's a last example showing you how to mount custom addons, how to use a custom configuration file and how to use volumes for the Odoo and postgres data dir:

version: '2'
services:
  web:
    image: odoo:12.0
    depends_on:
      - db
    ports:
      - "8069:8069"
    volumes:
      - odoo-web-data:/var/lib/odoo
      - ./config:/etc/odoo
      - ./addons:/mnt/extra-addons
  db:
    image: postgres:10
    environment:
      - POSTGRES_PASSWORD=odoo
      - POSTGRES_USER=odoo
      - PGDATA=/var/lib/postgresql/data/pgdata
    volumes:
      - odoo-db-data:/var/lib/postgresql/data/pgdata
volumes:
  odoo-web-data:
  odoo-db-data: