verify that dockerfile overwrites selected labels

[TomasTomecek]

If there is a label defined in base image and the dockerfile does not overwrite it, the resulting image may contain incorrect metadata.

This is a check only for dockerfiles.

List of labels:
summary
description
io.k8s.description (same as description)
io.k8s.display-name
io.openshift.tags
com.redhat.component
name
version

I fear that this check may be false positive in some scenarios (when the layered image contains only minor change which does not invalidate the metadata).