How to create custom Drupal image in Dockerfile?

I’m supposed to : Write a basic compose file for a Drupal content management system website.
This compose file should include ‘custom’ Drupal image for local testing and postgres image.

The only spcifications they gave about this Drupal custom image is:
-Use volumes to store Drupal unique data.
-Use ports to expose Drupal on 8080 so you can localhost:8080.

so, I don’t know what to write in this custom image.