"docker diff [container]" will show you what files have changed since the image was started, and you can then "docker cp" files out of the container to the host.

But for my part I tend to just layer changes piece by piece. So e.g. I have a docker image that's a "base development" image, and create separate docker containers to run each web app I'm experimenting with, with a shared volume with a separate docker container that I ssh into and run a screen session in with the code/git repositories. In effect that means that bringing up a new docker image for a new project is at most a couple of lines of change unless the project has particular/different dependencies.