I was trying to find a way to edit all files of my server remotely, setting "/" on the container's volumes as the root dir for the ftp. When I started the container, it changed the ownership of several folders in my file system to "staff". The biggest issue for me was losing the sudoers and the ownership of my own user folder, disabling me from using SSH. Despite my server being headless, I got to physically access the server and try to use the Ubuntu recovery to reset the ownership of crucial folders like sudo, but I had so many ruined folders that nothing worked. Long story short, I had to format and reinstall Ubuntu. Thank god I had a good backup, which saved me a lot of work.

I'm just reporting the issue, because I'm surely never gonna use this image after today, since it was very traumatic. I didn't even know a Docker container could be so catastrophic on my server.