When using Kaniko with certain build systems, such as GitLab CI, it is necessary to use the 'debug' image and override the entrypoint, so that an arbitrary build script can be executed within the container. Unfortunately, the 'warmer' binary is not available in the 'debug' image, making it impossible to perform a base image cache warmup using such build systems.
This patch addresses this by ensuring that the 'out/warmer' target is made in the initial stage, and that all 'out' files are copied to '/kaniko' in the final stage.
In the debug image, declare /busybox as a volume so that it is automatically whitelisted, because we don't want to delete it when building multi-stages images.
FYI this is required when using Jenkins, because we need to use the debug kaniko image to be able to start the container "doing nothing" (with /busybox/cat) before building (by executing /kaniko/executor directly inside the container)
See https://issues.jenkins-ci.org/browse/JENKINS-52576
Warren Seymour