gitlab: use custom docker:dind image

The current docker:dind container has broken default seccomp filter that
results in clone3 being blocked, which in turn breaks Fedora 35 rawhide.

This custom image has a workaround that causes the seccomp filter to
return ENOSYS for clone3 instad of EPERM, thus triggering glibc to
fallback to clone correctly.

Reviewed-by: Erik Skultety <eskultet@redhat>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

.gitlab-ci.yml

@@ -21,7 +21,8 @@ stages:
   stage: containers
   needs: []
   services:
-    - docker:dind
+    - name: registry.gitlab.com/libvirt/libvirt-ci/docker-dind:master
+      alias: docker
   rules:
     - if: "$TEMPORARILY_DISABLED"
       allow_failure: true