Software running in a container fails when using fontconfig?

[Simen Endsjø]

Hi, I'm packaging a piece of software which crashes when launching
chromium embedded framework.

The error is
  Fontconfig error: Cannot load default config file: No such file: (null)

I found a similar issue for Nix which suggests the error is caused by
fontconfig not being installed globally and suggests a patch:
https://github.com/NixOS/nixpkgs/issues/176081#issuecomment-1145903568

I tried creating a symlink at /etc/fonts/fonts.conf, but while I get
further, it still crashes.

  sudo ln -s ~/.config/fontconfig/fonts.conf /etc/fonts/fonts.conf

  Fontconfig warning: no <cachedir> elements found. Check configuration.
  Fontconfig warning: adding <cachedir>/var/cache/fontconfig</cachedir>
  Fontconfig warning: adding <cachedir prefix="xdg">fontconfig</cachedir>
  Trace/breakpoint trap

/var/cache/fontconfig is on a read-only filesystem, so I cannot
experiment with adding it there.

I tried adding all fonts and fontconfig to my system configuration, but
this didn't generate /etc/fonts/fonts.conf nor /var/cache/fontconfig/*.

Unfortunately, I haven't been able to test the Nix patch as my naive `-L
~/code/guix` fails to compile various modules.

Any input before I spent a lot of time trying to get this Nix patch working?