That was my initial attempt. Inside the docker, I found the file system for mine was kinda messed up that I found earlier. It was nested /uploads/uploads/uploads before /default/ was seen. Not sure what really happened, but I copied all files from the inside to my image mount, and added the mount folder as a volume.
Here, the situation was not that different from symlink. File permissions indeed create the same problem. After understanding that files are actually stored outside of Docker container, I thought symlink can be much easier solution.
For both, I am almost sure that it is file permission, but custom CDN just by Nginx server block sounds much more simpler solution that docker volume, as long as symlink doesnt work.