Custom lint rules

In large organizations or projects with multiple images, it would be ideal to enforce certain standards on all bootc images. Be it certain packages are installed/enabled, hardening rules are applied or simply that it follows conventions.

It would be nice if `bootc container lint` allowed the administrators to drop in custom linting checks, possibly in the form of bash scripts, which are evaluated at the end of the linting process. The output of those scripts determines the result of `bootc container lint`. Since these scripts are stored inside the image, they will be present on downstream images too which makes these lints easy to propagate.

I'm proposing a new directory containing custom linting rules on the image, `/usr/lib/bootc/lint.d/`, which `bootc container lint` evaluates when called. This could be Bash or a proprietary syntax.

This isn't difficult to build something custom on your image, though I feel it would be beneficial to bake something into the existing linting action.