Make the CI pipeline robust vs non-ASCII characters in the environment

As you can see, the CI pipeline passes with these changes, even though I'm using the real spelling of my last name again for the GitLab account. :)

Generating the docker image as part of the pipeline is based on xorg/driver/xf86-video-amdgpu!16 (merged).

v2 (only patch 2 needs to be reviewed again):

• 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.