ci: Unset user and working dir in toolbox image

The mutter image now uses a default user that doesn't exist outside
the container, which makes the image unusable for toolbox. Undo the
damage by unsetting the user again when building the toolbox image.

.gitlab-ci/build-toolbox-image.sh

@@ -69,6 +69,7 @@ build_container() {
 	EOF
   buildah copy --chmod 755 $build_cntr $update_mutter /usr/bin/update-mutter
 
+  buildah config --user "" --workingdir / $build_cntr
   buildah config --env HOME- \
     --label com.github.containers.toolbox=true \
     --label org.opencontainers.image.base.name=$MUTTER_CI_IMAGE \