Add stage for building image with Kaniko

The main motivation for this is:

• Docker-in-Docker requires privileged mode to function, which is a significant security concern.
• Docker-in-Docker generally incurs a performance penalty and can be quite slow.

Note that we need to use the debug image of Kaniko so we get a shell.

Ref: https://docs.gitlab.com/ee/ci/docker/using_kaniko.html