Docker/oci and thus gitlab-runner, default to a root
user inside the namespace, even if its an unprivileged
one.
This can cause issues and let permission bugs sneak in,
as we are functionally root when running the build.
Switch the build jobs to run with our new "containeruser"
so we avoid much of it.
Our user is still in the wheel/sudo group but that's fine
as long we don't elevate the privileges unintentionally.
Noticeably for the time being, we will need to chown the
CI_PROJECT_DIR checkout as the gitlab runner might try
to reuse pre-existing and cached volumes of the project
checkout.
Additionally we need to change the ccache path, so we
will avoid the existing cache owned by "root".
Close https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/2433
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8763>
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