Nextcloud
Manually adding files
It is possible to add files directly in the server, by transfering them in the Nextcloud's app volume. The volumes are located under /var/lib/docker/volumes, and the files are in the data/__groupfolders/1/ subfolder of the volume.
To correctly add the files, you may need to convert the filename encoding, the easiest being using rsync. For example: rsync -ahr --info=progress2 --iconv=ISO-8859-1,utf8 /var/gdrive/03_Événementiel/ /var/lib/docker/volumes/nextcloud_nextcloud-app/_data/data/__groupfolders/1/GDrive/03_Événementiel/.
Once added, you need to index the files in Nextcloud for them to be available in the web application. To do so, run the command php ./occ files:scan --all from the /var/www/html directory as the user www-data within the Nextcloud's container. The easy way is to get the container's name using docker ps, then run docker exec -it -u www-data <CONTAINER> php ./occ files:scan --all. This may take a while, so I recommend to use screen to start it in background, such that you can close the connexion without stopping the indexing.