Contents of folder are not being copied to mounted volume

My understanding is that when you build a container with mounted volume when you start the container and mount said volume, the folder content from the container is supposed to be copied to the mounted volume.
Is my understanding correct?

The reason I ask is because I have created an Nginx container and I want to mount the contents of /etc/nginx to my mounted volume.

I run it: docker run -d -v /config:/etc/nging -p 80:80 my-nginx