Add option to pull a new image

[unixorn]

Description

We can now pull new images, not just pull existing ones.

Closes #17

Checklist

• All new and existing tests pass.
• Any added/updated scripts added use #!/usr/bin/env interpreter instead of potentially platform-specific direct paths (#!/bin/sh and #!/bin/bash are allowed exceptions)
• Added/updated scripts are marked executable
• Scripts do not have a language file extension unless they are meant to be sourced and not run standalone. No end-user should have to know if a script was written in bash, python, ruby or whatever. Not including file extensions makes it easier to rewrite the script in another language later without having to change every reference to the previous version.
• I have confirmed that the link(s) in my PR are valid.

License Acceptance

• This repository is Apache version 2.0 licensed (some scripts may have alternate licensing inline in their code) and by making this PR, I am contributing my changes to the repository under the terms of the Apache 2 license.