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:

    sudo docker run busybox echo "fun" > newfile

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

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

Enter the container ID returned: