gitlab ci: docker: switch to overlay2 driver

see <https://blog.sparksuite.com/7-ways-to-speed-up-gitlab-ci-cd-times-29f60aab69f9>

gitlab ci: docker: switch to overlay2 driver
12d3aa0a · nrbffs · 852a88af · 12d3aa0a
Commit 12d3aa0a authored 5 years ago by nrbffs
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -2,6 +2,7 @@ image: registry.gitlab.freifunk-stuttgart.de/firmware/ffs-buildenv:master

 variables:
  GIT_SUBMODULE_STRATEGY: normal
+  DOCKER_DRIVER: overlay2

 stages:
  - build