lint mode

[ghostsquad]

I'd like to run this in CI as a linter and have some options & useful output to show me the packages/images that aren't pinned.

Requiring the developer to make a change locally and decide how to pin (either using this and automatically updating) or choose a version manually.

[deadnews]

If this is still relevant, https://github.com/deadnews/pindock has a check command for it.