The same way we can mount a volume in a service, I would like an option to mount a static config, where I can paste a config in a box and provide the path where I want this config to be mounted.

This is similar to the config feature of docker compose:
https://docs.docker.com/reference/compose-file/configs/

A concrete example would be a caddy service, where I can start a service based on the official image then mount a Caddyfile through the railway UI. Bonus points if I can use railway variables in the config.

The feature would be the same as mounting a volume, except there could be a tab where I can edit the config file with indentation etc...

Currently to achieve this I have to host a gh repo with a Dockerfile and the config file where I only have a COPY command in the Dockerfile and use this for my deployment instead. While this is ok for personal use, when used in a template where multiple service are configured together, it would be cleaner to be able to do this from the app itself.