there are many different way to run Nextcloud and Docker desktop is definitely not the most common one and not a preferred one. Looking for beginner friendly guides I would recommend you choose more common installation ways. And at the end you as an admin must familiarize yourself with the technology you are administering otherwise you will end up in trouble sooner or later. 101: Self-hosting information for beginners
with the regular Docker the command would be docker exec --user www-data nextcloud-aio-nextcloud php occ user:resetpassword admin I think it is similar on docker-on-windows. The command should run as webserver user ( --user www-data) and the executable itself is php and occ ... is an argument.
UPDATE: how-to-run-occ-commands in AiO
Simply run the following:
sudo docker exec --user www-data -it nextcloud-aio-nextcloud php occ your-command.
Of courseyour-commandneeds to be exchanged with the command that you want to run.