One time run? Or does it have to happen every rebuild? One time directions:
On your droplet:
cd /var/discourse
cp YOURSCRIPT shared/
./launcher enter app
And you are inside the container. Your script is now available as /shared/YOURSCRIPT
That sort of thing is good for one time DB tasks (where the results are stored in the DB) or stuff that runs a report but doesn’t need to be there after the next upgrade. For stuff that needs to be around always, you have to work with the app.yml file.
(And backups are always good.)