This application is used for many of the exercises in https://devopswithdocker.com as an application which students can use to test out different features of containers.
These include the following. Bolded is the feature of the app and after it the learning experience:
App accepts arguments
CMD docker run XXX command
App generates content to a file
docker exec docker cp volumes
App starts a web server
docker run -p volumes
App accepts PORT env and respects it
heroku deploy