/var/discourse/shared/standalone/backups/default appears to have compressed backups of all the important bits, the database and uploads.
Thank you for the ideas, but it was a no-go.
Should I simply try reinstalling Docker, or if I do that, I might as well nuke the whole thing and do a complete fresh install of Discourse entirely (I installed via Discourse | DigitalOcean Marketplace 1-Click App)?
I assume importing the backup file is simple to do in the Discourse admin?