title: Cache issue when building a multi-stage image with kaniko
body: <p>I've setup container images builds in kubernetes using kaniko (version <code>v1.6.0-debug</code>), but I ended up in the following problem when the cache is enabled (<code>/kaniko/executor --cache --cache-repo $CI_REGISTRY_IMAGE/cache</code>).</p>
  <p>This is a simplified Dockerfile that explains the issue (the real one has a PHP <code>package.json</code> and <code>package.lock</code> files where new requirements have been added).</p>
  <pre><code>FROM busybox:1.33.1 AS builder
  WORKDIR /root
  COPY testfile testfile
  
  FROM busybox:1.33.1 AS release
  WORKDIR /root
  COPY --from=builder /root/testfile testfile
  RUN cat /root/testfile
  </code></pre>
  <p>If <code>testfile</code> is modified, and the docker image was previously built (and thus all its layers cached in the docker registry), kaniko does not see any modification and uses the cached layer for</p>
  <pre><code>COPY --from=builder /root/testfile testfile
  </code></pre>
  <p>The result is that the final docker image provides an old (cached) version of the file <code>testfile</code> instead of the last one.</p>
  <p>Any idea how to solve/mitigate this problem? Of course if I remove the cached layers from our docker repository, the image is build correctly, but disable the cache is not an option for me because the builds would be too slow.</p>