How to create custom docker template with image exposing a single file?

[Crovaxon]

Hello folks,

I have tried to get a custom added docker container to run and I know the basics but I have no experience at all with setting up custom non-CA images.
I've given up on this before with other containers that expose single files but with my most recent attempt I got fed up enough to ask for help  This is what I am currently trying to run: https://github.com/SillyTavern/SillyTavern/blob/main/docker/docker-compose.yml
The container exposes the config path and a file as volumes.
I have no trouble mapping a path for the config volume to the respective appdata path for this docker.
But the second exposed volume is a single file and I have no success mapping this via the path option in the Unraid UI. How would I go about getting this into my appdata?
Seeing that both the config folder and this config.conf file reside in the parent directory "app" I tried to add a mapping between the "app" folder and a folder in appdata instead, hoping to just fully expose this folder instead, but then the container fails to start at all:
 

[FATAL tini (7)] exec ./docker-entrypoint.sh failed: No such file or directory

So this wasn't it either.

What is the correct approach here? Is there a way to map the file as well into the appdata directory or can I alternatively just expose the parent folder somehow without the container complaining, or would this require rewriting the compose file?