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Docker compose can be used to run multiple containers at once. For example, if you want to set up something like a WordPress site you will need a web server and a MySQL database. We can use the docker-compose.yaml file to define all these services and their behavior.

Note: Ideally you would just get a WordPress container image that has all of this already installed. This example is used just to demonstrate how to use docker compose.

#Specify the docker version we are using.
version: "3"

#Define services/containers.
services:
  #1. - Web server.
  web:
    #Specify base image. Could be your image or one from docker hub.
    image: "php:7.4-apache"
    #Connects ./src from project folder to /var/www/html folder within the container. 
    volumes:
      - "./src/wordpress:/var/www/html/wordpress"
    #Restart on crash.
    restart: 'always'
    #Specifies that the DB should be running before the web server.
    depends_on:
      - db
    #Maps internal docker ports to outer port.
    ports:
      - '8080:80'
    #Specifies connection with the DB service.
    links:
      - db
    networks: #Connects this container to myNewNetwork network.
      - myNewNetwork  
  
  #2. - Database.
  db:
    #Specify base image.
    image: mysql:latest
    #Restart on crash.
    restart: 'always'
    #When a container is shut down it will lose all it's data.
    #We can define volumes a wher we can store data Persistance 
    volumes: 
      - ./src/dbData:/var/lib/mysql
    #Define environmental variables that will be passed to the container when it runs.
    environment: #${ someVariableName } can be used to define a variable you will need to pass with the docker-compose command.
                 #This way whoever uses this can easily set their own user./pass. without changing the docker-compose file.  
      MYSQL_DATABASE: "wordpress"
      MYSQL_USER: "user" #${USERNAME} <== example of how it can be used.
      MYSQL_PASSWORD: "somepassword" #"${MYSQL_PASSWORD}" <== example how it can be used.
      MYSQL_ROOT_PASSWORD: "somepassword" #"${MYSQL_PASSWORD}" <== example how it can be used.
    networks: #Connects this container to myNewNetwork network.
      - myNewNetwork  
    
#Here we can define our networks.
networks: 
  myNewNetwork: #Define a new network.
    driver: bridge #Sets bridge mode(similar to when you setup VMs).