Docker Compose introduces the concept of multi-container applications using an all-in-one application components' definition file. This file is known as docker-compose.yaml. Usually, we will manage a docker-compose.yaml file. Notice that this is a YAML file; therefore, indentation is fundamental. The file will contain all of the application components and their properties.

This is how a simple docker-compose.yaml file looks (we can use either the .yaml or .yml extension for YAML files):

version: "3.7"
services:
  lb:
    build: simplestlb
    image: myregistry/simplest-lab:simplestlb
    environment:
      - APPLICATION_ALIAS=simplestapp
      - APPLICATION_PORT=3000
    networks:
      simplestlab:
          aliases:
          - simplestlb
    ports:
      - "8080:80"
  db:
    build: simplestdb
    image: myregistry/simplest-lab:simplestdb
    environment:
        - "POSTGRES_PASSWORD=changeme"
    networks:
       simplestlab...