* Allow to disable the fallback to the default registry on image pull
When one or more registry mirror(s) are deffined with the 'registry-mirror' argument, if none of those mirrors include the image,
the current behavior is to fallback to the default registry.
If a whitelist (or some image restriction) is applied at the mirror side, fallbacking to the default registry makes that restriction useless.
This new argument allows to skip the fallback and abort the build if the mirror rejects an image.
If it is not set, is completelly transparent.
* fix typo on command help
c2445c76da