Thank you so much for your kind effort. I tried just now and the problem has perfectly solved. But it seems there is another problem:
I specified the share path for this container as above: /mnt/user/Documents/
When I log in through webdav remotely, I did have access to the webdav server, but it was empty. Then I carefully checked everything and found that it seemed that this container created a new folder (/mnt/user/Documents/data) under Documents where I specified, which means that I can only have access to /mnt/user/Documents/data instead of what I specified (/mnt/user/Documents/)
I'll be so grateful if you may tell me how to fix it. I do need to have access to /mnt/user/Documents/ through webdav.
Thank you so much!