Allow kaniko caching in separate image repository

Proposal

It would be really helpful to allow kaniko layer caching in a separate image repository within the same image registry (eg image/repository/path/cache). When the --cache flag is set, kaniko appends cache to the end of the repository set as the --destination. For example, if the --destination is set as image/repository/path, then kaniko will attempt to cache layers in image/repository/path/cache.

As it stands now, this setting results in a 401 unauthorized from gitlab. To get around this, you have to set the --cache-repo flag to be equal to the --destination (ie --destination=image/repository/path --cache-repo=image/repository/path). While this works, it's less than ideal when combined with automated repository cleanup. Depending on the image (and the number of layers in the image) the cached layers end up taking up most of the allowed space in the image repository, limiting the number of actual images available to only 1-3. The cached layers also behave differently with respect to automated repository cleanup than a normal image would. For example, you wouldn't necessarily want a labor intensive layer that is rarely changed to be deleted from the cache to make room for less labor intensive layers that are changed frequently. Whereas, this kind of approach would be perfectly reasonable for normal images where you don't really need to retain older versions of an image.