Docker compose example

docker-compose.yml

# we are using the v3.7 syntax to write this docker compose file
version: "3.7"

# define a list 
networks:
    backend:

# the services section tells docker compose how to run the docker images
# a service can have multiple containers, all based on the same image
services:
    # there are 2 ways to build a service
    # 
    # 1. extends from a Dockerfile using the 'build' option
    # 2. use an existing image from dockerhub using the 'image' option
    # In our example here, php-app uses the 'build' option while mysql uses 'image'
    
    # we define the 'php-app' service
    # you can name this to anything you want
    php-app: 
        build: 
            # assume there is a Dockerfile exist in the specified dir
            # context is a suboption of 'build'
            context: ./php-app  # path to the docker folder containing Dockerfile

            # an alternate dockerfile to build this image
            # dockerfile: Dockerfile-alt 

            args: # arguments supply to the docker file
                - PHP_VERSION=7.4.1-fpm-buster

        # port forwarding - binding host port to container's port
        ports: 
            - "3001:80"  

        # list of directories to set up as volumes
        volumes: 
            - "../src/index.php:/var/www/html/index.php"

        # the starting working path when 'ssh' into docker container
        working_dir: "/app" 

        # the php-app container will only run once the services below are ready
        # in this case, php-app is only dependent on mysql
        depends_on:
            - mysql 

        # the networks that php-app will connect to
        # the network name corresponds to the top level network key defined above
        networks:  
            - backend
        
        # exposing a port to internal network. 
        # They will only accessible to linked services, ie within the same network
        # in this case, the port will be exposed to the 'backend' network
        expose: 
            - 80
            - ${PHP_PORT} # env variable read from .env
        
        # list of hostname mapping, added to container's host file entry
        extra_hosts: 
            - "testsite.dev.local:0.0.0.0"
        env_file: # path of env file to reference env var in dockerfile
            - "./.env"
        
        environment: # adding one-off env vars
            - PHP_IDE_CONFIG=vscode  # env var to pass to container

        # same as cmd in dockerfile. Will override the default command
        command: "echo hello!!" 
        # or list form:
        # command: ["echo", "hello!!!"]

        # we can supply a script to entrypoint
        entrypoint: ./entrypoint.sh  # same as entrypoint in dockerfile
        # or list form:
        # entrypoint: ["echo", "hello"]

    
    # defining the mysql service
    mysql:
        # using prebuild image
        # if we have the 'build' and 'image' option simultaneously
        # docker will build the image from the dockerfile, and tag the image with
        # value provided in the image option
        image: 'mysql:8.0.21'
        networks: 
            - backend
        expose: 
            - 3306