Add support for hermetic mode, where registry is never contacted and all images expected to be preResolved

[redbaron]

Describe the problem/challenge you have

when used together with ytt, kbld must be applied after templating is done , which often happens just before the deploy. If some images are missing from the imageLock file, then they are implicitly resolved, leading potentially to a non reproducible deploys and arguably defeating the purpose of this tool.

Describe the solution you'd like
Add a new flag --hermetic=true, which makes kbld fail when it encounters not preresolved image.

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[cppforlife]

i like this suggestion!