How to add custom graby-site-config to Wallabag docker version?

How can I add a custom graby-site-config to the docker version of Wallabag? `/var/www/wallabag/vendor/j0k3r/graby-site-config` has many files in it, so I can't map it to a host directory and add my custom file there. But I can't add it directly to the image, of course, or it won't persist.

Start by reviewing the Docker image and its mount configuration around /var/www/wallabag/vendor/j0k3r/graby-site-config. Determine how a custom file can persist without hiding the existing directory. Done means documenting a supported way to add custom graby-site-config files to the Docker deployment.

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