Warmer does not use existing cache

[valentinsoe]

I am experimenting with kaniko in Gitlab CI pipelines. And I use caching extensively to provide faster pipeline execution. So I've tried to use kaniko-warmer for faster docker builds.
The problem is currently kaniko-warmer downloads images regardless of cache directory state.

I propose kaniko-warmer to check image existence in the cache directory prior to downloading it.

This will require minor code modifications. PR with them is coming.

[valentinsoe]

I've implemented cache reusing in warmer and submitted PR.