[Feature] Add packages without changing the template

[CC007]

It would be nice to be able to add packages without having to change the template. That way when the template gets updated (like an update to the base image or to the predefined package bundles) I don't have to update my edited template.

So instead of specifying a build_option bundle like dev, which is hardcoded in the template, it would be nice to be able to specify your own extra packages to install, similar to how you can specify which python packages to install.