Custom Nginx config file


So I want to use a custom Nginx config file which I used to mount into my container on my dev machine. I know there is no mounting support on your side only named values.
I suppose the reason is that you build the image and ship it without the files from the repo where my Nginx config files reside. Is that right?
So all I need for the container to be executed on the device must be built into the image. Correct?
So how does one solve the issue where a custom config file, e.g. for Nginx is used? Building a custom image?