Here is my docker compose file below. I think I used the standard file that the developer ships, simply because I was keen to get firefly going without fully understanding the complexity of docker storage in volumes.

 The Firefly III Data Importer will ask you for the Firefly III URL and a "Client ID".
# You can generate the Client ID at http://localhost/profile (after registering)
# The Firefly III URL is: http://app:8080/
#
# Other URL's will give 500 | Server Error
#

services:
  app:
    image: fireflyiii/core:latest
    hostname: app
    container_name: firefly_iii_core
    networks:
      - firefly_iii
    restart: always
    volumes:
      - firefly_iii_upload:/var/www/html/storage/upload
    env_file: .env
    ports:
      - '84:8080'
    depends_on:
      - db
  db:
    image: mariadb:lts
    hostname: db
    container_name: firefly_iii_db
    networks:
      - firefly_iii
    restart: always
    env_file: .db.env
    volumes:
      - firefly_iii_db:/var/lib/mysql

  importer:
    image: fireflyiii/data-importer:latest
    hostname: importer
    restart: always
    container_name: firefly_iii_importer
    networks:
      - firefly_iii
    ports:
      - '81:8080'
    depends_on:
      - app
    env_file: .importer.env

  cron:
    #
    # To make this work, set STATIC_CRON_TOKEN in your .env file or as an environment variable and replace REPLACEME below
    # The STATIC_CRON_TOKEN must be *exactly* 32 characters long
    #
    image: alpine
    container_name: firefly_iii_cron
    restart: always
    command: sh -c "echo \"0 3 * * * wget -qO- http://app:8080/api/v1/cron/XTrhfJh9crQGfGst0OxoU7BCRD9VepYb;echo/" | crontab - && crond -f -L /dev/stdout"
    networks:
      - firefly_iii
volumes:
   firefly_iii_upload:
   firefly_iii_db:

networks:
  firefly_iii:
    driver: bridge