docker-compose.yml

# docker-compose version : the latest and recommended version, cross compatible between compose and Docker Engine's swarm mode
version: '3'

# services : define the services that are going to be instantiated with docker-compose
services:

 # defining our flask web application
 flaskapp:

   # build the Dockerfile that is in the web directory: http://bit.ly/2BpBPXq
   build: ./web

   # always restart the container regardless of the exit status; try and restart the container indefinitely
   restart: always

   # expose port 8000 to other containers (not to the host of the machine
   expose:
     - "8000"

   # mount the web directory (http://bit.ly/2BbJO6k) within the container at /home/flask/app/web
   volumes:
     - ./web:/home/flask/app/web

   # don't create this container until the redis and postgres containers (below) have been created
   depends_on:
     - redis
     - postgres

   # link the redis and postgres containers together so that they can talk to one another
   links:
     - redis
     - postgres

   # pass environment variables to the flask container (this debug lets us see more useful information)
   environment:
     FLASK_DEBUG: 1

   # deploy with 3 replicas in the case of failure of one of the containers
   deploy:
     mode: replicated
     replicas: 3

 # defining the redis docker container for our web application
 redis:

   # use the redis:alpine image: https://hub.docker.com/_/redis/
   image: redis:alpine
   restart: always
   deploy:
     mode: replicated
     replicas: 3

 # defining the redis NGINX forward proxy container for our web application
 nginx:

   # build the nginx Dockerfile: http://bit.ly/2kuYaIv
   build: nginx/
   restart: always

   # Expose port 80 to the host machine
   ports:
     - "80:80"
   deploy:
     mode: replicated
     replicas: 3

   # Our flask application needs to be available for NGINX to make successful proxy requests
   depends_on:
     - flaskapp

 # defining our postgres database for our web application
 postgres:
   restart: always
   # use the postgres alpine image: https://hub.docker.com/_/postgres/
   image: postgres:alpine

   # Mount an initialization script and the persistent postgresql data volume
   volumes:
     - ./postgres/init.sql:/docker-entrypoint-initdb.d/init.sql
     - ./postgres/data:/var/lib/postgresql/data

   # Pass postgres environment variables
   environment:
     POSTGRES_PASSWORD: linode123
     POSTGRES_DB: linode

   # Expose port 5432 to other docker containers
   expose:
     - "5432"