ci/test-clang.sh: Add 'z' option to '--volume'

When SELinux is enabled, such as on Fedora, proper labels are needed,
otherwise the tests cannot start:

[+] Testing clang in 'docker.io/fedora:latest' container
bash: /repo/ci/test-clang-docker.sh: Permission denied

This is supported by both Docker and Podman.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>

ci/test-clang.sh

@@ -31,7 +31,7 @@ for docker_image in "${docker_images[@]}"; do
     echo "[+] Testing clang in '$docker_image' container"
     "$docker" run \
         --rm \
-        --volume "$rootdir":/repo:ro \
+        --volume "$rootdir":/repo:ro,z \
         "$docker_image" \
         bash /repo/ci/test-clang-docker.sh
 done