Treat "apt-get clean is not used" as an error

[mcandre]

As an engineer, I want dockerfile_lint to exit with a non-zero status when apt-get commands are referenced, but apt-get clean is never referenced, so that I can proactively reduce the size of my images.

Currently, dockerfile_lint reports this finding as an "INFO"-level finding. I would prefer that this either be considered a WARN-level finding. Or, have some other way to configure dockerfile_lint to treat certain INFO-level warnings as errors.