ok, that’s not your problem then. Only one container and only 0.5MB of logs.
if you run du -sh * again in the / top directory maybe you will get a better picture of where the space has gone. I highly recommend ncdu to make it easier ( NCurses Disk Usage )
on my test instance docker uses about 8GB on images, check out Prune unused Docker objects | Docker Documentation for help on how to clean that up.
docker image prune -a usually frees up a lot of space for me, you might have to
docker rm app to delete your old Discourse app container first. If you have lots of forum backups maybe you could download them to your local machine to free up some additional space.
another thing you should check is
docker ps -a to see if you have any unexpected containers. Sometimes a failed rebuild will leave a dangling container that you have to remove manually.
[root@discocent /]# docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
86981cd18ab1 da251a932851 "/bin/bash -c 'cp -r…" 4 days ago Exited (0) 4 days ago festive_margulis
[root@discocent /]# docker rm festive_margulis
[root@discocent /]# docker image prune -a
WARNING! This will remove all images without at least one container associated to them.
Are you sure you want to continue? [y/N] y
Total reclaimed space: 2.33GB