give the option to persist the database to the host

docker-compose.yml

@@ -21,6 +21,9 @@ services:
     restart: always
     volumes:
       - db:/var/lib/mysql
+      # If you want to persist data on the host, comment the line above this one...
+      # and uncomment the line under this one.
+      #- ./docker-compose/db/data:/var/lib/mysql:rw,delegated
     environment:
       MYSQL_USER: crater
       MYSQL_PASSWORD: crater