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.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>