Alles anzeigenYou need to bash into the container to run that command.
docker exec -it nextcloud bash (assuming your container is named nextcloud)
or
In Portainer, click the >_ icon next to your nextcloud container, then click connect. This will open a sort of terminal window with you bashed into the container.
Either way, once you are bashed in, run the command
Just used this to run occ db:add-missing-indices and it worked a treat - thank you!