gitlab-ci: Docker image can be generated as part of pipeline

This removes the dependency on an externally generated docker image, and
should make it easier to update the docker image or make other changes
related to it.

This is based on Debian testing, because I'm most familiar with Debian.
But it should be easy to base it on another distro.

v2:
* Use kaniko instead of docker-in-docker for image generation, so it can
  also work in unprivileged runners.
* Drop piglit.conf & tetexec.cfg overrides, just make sure the files in
  the image work.
(Cherry picked from commit f56d8e22)