Best to start by measuring and sharing your measurements - you might well be right about the best way to proceed, but let’s see the shape of things.
Although the docker prune is a good idea too.
The size of the disk:
df -h /
And the usage of that disk right now:
And the usage by docker:
docker system df
docker images -a
docker image ls
docker volume ls -qf dangling=true
(I’m not a docker expert - that’s probably a bit redundant, but with all of those outputs we should be able to see the docker use)
Overall, your disk usage will be a combination of
- the operating system
- swap files
- various log files
- your discourse software
- your backups
- your database
- your forum uploads
For many forums, the uploads are the big chunk. But one must measure!
If you’re not an expert linux sysadmin, there will probably be some extra unneeded older things too.