What is tianon/true used for in Dockerfile?

I come across some docker and found tianon/true image that used in my docker file.

My docker-compose.yml file look like this :

app:
  image: mageinferno/magento2-nginx:1.11-1
  links:
    - phpfpm
    - db
  volumes_from:
    - appdata
  ports:
    - 8000:80
  env_file: env/nginx.env

appdata:
  image: tianon/true
  volumes:
    - /var/www/html
    - ~/.composer:/var/www/.composer
    - ./html/app/code:/home/gujarat/php/html/app/code
    - ./html/app/design:/home/gujarat/php/html/app/design
    - ./html/app/etc:/var/www/html/app/etc
    - ./html/downloads:/var/www/html/downloads

phpfpm:
  image: mageinferno/magento2-php:7.0-fpm-1
  links:
    - db
    - mail
  volumes_from:
    - appdata

db:
  image: percona:5.7
  volumes_from:
    - dbdata
  ports:
    - 8001:3306
  env_file: env/mysql.env

dbdata:
  image: tianon/true
  volumes:
    - /var/lib/mysql

setup:
  image: mageinferno/magento2-php:7.0-fpm-1
  command: /usr/local/bin/mage-setup
  links:
    - db
  volumes_from:
    - appdata
  env_file: env/setup.env

What I don't understand is what is tiano/true is used for ? From above setup this image is used appdata

I found this link on github but no readme at all on the first page.