the solution that works for me is to change the place mnt/ram_disk is added to the container. It should not be mnt/ram_diskbut some place in the directory tree containing the web pages.

I added a bind to add /mnt/ram_disk to /var/www/html/ram_disk, and with that, I cann access any image inside that folder properly.

Thanks to Julien B. for suggesting that in a different thread.