Thanks for your reply. Hopefully this will help.
The only directory I mapped in the the Docker template is "/mnt/user/Software/"
This is an Unraid share named "Software", which is shared with others.
I told Steam-headless container to use this directory for steam games: /mnt/user/Software/RTP Games/Steam/
When a user browses the "Software" share, they can see all files on all devices in my Unraid system. Effectively exposing all files on the system to anyone with access to that share. They can open and edit.
So they can just just browse all the way to:
Software\RTP Games\Steam\steamapps\compatdata\1493710\pfx\dosdevices\ZSXKM0~L\mnt\disk1
Software\RTP Games\Steam\steamapps\compatdata\1493710\pfx\dosdevices\ZSXKM0~L\mnt\disk2
Software\RTP Games\Steam\steamapps\compatdata\1493710\pfx\dosdevices\ZSXKM0~L\mnt\cache
etc
Everything between "\Steam\" and "\mnt\" was generated by this container.
\mnt\disk1,2,3 cache, etc, is every drive plugged into my system.
You can then open these folders and see everything on every disk.
I had the Nvidia Available devices setting set to "all" before. I JUST changed it and specified the GPU, and it didn't make a difference. But these links work when the container was not running, so it probably wouldn't help now.
If I look at the file properties of this folder:
Software\RTP Games\Steam\steamapps\compatdata\1493710\pfx\dosdevices\ZSXKM0~L\mnt
my computer thinks it is much bigger than my total storage space, lol!
Hopefully this helps clarify the issue.
Whatever happens, thanks for your work on this container. It's the only way I managed to run any games on this system, after wasting far too many weekends trying to pass through a GPU to a VM.