All the following commands are executable also through the container. Assuming your application container is nextcloud-app, for instance, you should run the following from the DSM shell.
docker exec --user www-data nextcloud-app php occ <ommand here>-
Scan & cleanup
occ files:scan --all occ files:cleanup -
Set maintenance mode
occ maintenance:mode --[on|off] -
Check setup
occ integrity:check-core -
Upgrade to a newer version (no need using containers)
occ upgrade -
Get the current configured trusted_proxies
occ config:system:get trusted_proxies