Update Kaniko image to latest

We have not updated the Kaniko image in a long time and a set of issues have surfaced which seem to be solved in the latest debug version.

Fixes cache warning

Fixes #4 (closed)

NB: a backup of the latest tag used since May 2019 was done as gitlab-registry.cern.ch/ci-tools/docker-image-builder:backup_before_kaniko_update, so it is possible to revert this change by restoring the previous latest tag.