The first step to creating an image is to create a container and fill it with some data.

First we'll create a container with a single new file based off of the ubuntu base image:

docker run ubuntu echo "fun" > newfile

The container will immediately terminate (because its one command is echo), so we'll use docker ps -l to list it:

docker ps -l
CONTAINER ID        IMAGE               COMMAND             CREATED 
07f2065197ef          ubuntu:12.04        echo fun            31 seconds ago