where should I put my config file?

[Goffi]

Hello,

to adapt to flatpak, I need to set a configuration file to indicate path to 
media files. This file should be in /etc on a classical distribution.

It seems that I can't write to /etc (I can do it in the manifest, but the 
file is not there after building).

Is there a way to write persistently to /etc, or should I modify my 
software to check /app to find configuration file?

I'm excluding putting my config in $HOME on in xdg's config directory, 
because I'll probably have to give access to host's $HOME for some features 
(like file sharing).

Thanks
Goffi