Add new Kaniko Dockerfile for PDNS builder

[aerique]

(I'm not sure @wojas needs to review this. Possibly the Kaniko build process was added after @pieterlexis took ZoneControl over.)

A custom Kaniko image for PDNS Builder, because the official Kaniko debug image only offers Busybox and the PDNS Builder also needs Bash, Git, Perl, rsync and perhaps more.

For now we put it besides Dockerfile-kaniko until it is clear that one can be removed.

AFAIK this file is only used for ZoneControl builds but I'm not entirely sure, that's why I do not want to remove Dockerfile-kaniko yet.

This new Dockerfile has been successfully used for the latest ZoneControl builds as part of the process to fix the flaky builds.

It is available at https://hub.docker.com/repository/docker/powerdns/kaniko-pdns-builder/ which is where the ZoneControl builds pull it from.