docker-compose.yml

version: '3'
# Keep in mind the version number.

services:
# Each service here (flask and nginx) is a different container.
# The arguments (build, environment, etc) can also be passed over the command line
# with docker run. docker-compose allows us to define them with more 
# consistency.


# Services defined here all run in a network of containers which have some ability
# to pass information between them. See expose.

    flask:
        build: flask
        # tells docker-compose where to find Dockerfile for the flask
        # container we are building. 
        # (in this case <location of docker-compose.yml>/flask/Dockerfile)
        environment:
        # Environment variables for the flask container.
            - PORT=8000
            # Defines port for the flask app to run on (in container)
            - FLASK_APP=app.py
            - PYTHONUNBUFFERED=0
            # PYTHONUNBUFFERED makes it so that you can see python's error
            # output in the container's logs.
        expose:
        # The expose command makes it such that other containers in the network
        # can see the given port of this container. In our case, nginx container
        # can access http://flask:8000 . Expose DOES NOT reveal the port 
        # outside of the container network. Therefore, you will not see the app
        # running on port 8000 of your local machine (localhost:8000).
        # Nor will you see if from your system on flask:8000. It can only be 
        # seen by the other containers in the network.
            - 8000
            # Also note that it is written "- 8000". NOT "-8000".
            # Put a space after the dash for all arguments in the docker-compose
            # file or you might have issues getting it to work.
        command: flask run
        # Overrides command in Dockerfile.
    
    nginx:
        build: nginx
        ports:
        # Ports (unlike expose) make the container's ports accesible outside
        # of the container network. This command allows us to see port 80 from
        # your system. 
            - 80:81
           #- <system port>:<container port>
           # In other words, map port 81 in the nginx container to port 80
           # on the system.