Hey there. I don't have the exact same issue, as I got a message about not having permissions due to root being the owner. So all I had to change ownership via
sudo chown -R USERNAME /opt/vintagestory
chown changes the owner of the target, USERNAME selects the new owner, /app/extra/vintagestory is the target directory, and -R changes permissions of all files and directories within the target. Sudo is needed because the owner is root, and higher privileges are needed.
I do not know how read-only filesystems work in linux. you may be able to get it to work by using my command (replacing the path with /app/extra/vintagestory), and then doing chmod -R u+w /app/extra/vintagestory, possibly with sudo.
Be careful though. Back up your saves and all that.
