When docker volume specifies a single file, after the host modifies the file content, the file content in the container does not change

  docker, question

Did docker intentionally design this? Why?
I think it should be hosted on the host like a shared folder. After modification, the contents in the container should also change accordingly.
Did I think wrong?
If the invariance in container is right? What is the design intent?

It is suggested to use docker volume to map the folder of the host. File changes in the host will be reflected in the container, but the service in the container will not stop.