[librsvg-oci-images/alatiera-master-patch-60525] gitlabci: Use overlay2 as docker driver

[Jordan Petridis <jpetridis src gnome org>]

commit 4d2f3159b250e0c59186e8491a4f6199f0cd7395
Author: Jordan Petridis <jordanpetridis protonmail com>
Date:   Fri May 18 18:57:43 2018 +0000

    gitlabci: Use overlay2 as docker driver
    
    Many thanks to jjardon for this.

 .gitlab-ci.yml | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 550fe32..b5e6281 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -2,6 +2,11 @@ image: docker:latest
 services:
     - docker:dind
 
+# When using dind, it's wise to use the overlayfs driver for
+# improved performance.
+variables:
+    DOCKER_DRIVER: overlay2
+
 stages:
     - distro
     - rustc