Make cgroup mount not readonly, needed for newer Docker for Mac setups.

Fixes DOMjudge/domjudge-packaging#170

(cherry picked from commit 0a72de8)

Author: nickygerritsen

docker-compose.yml

@@ -22,7 +22,7 @@ services:
     image: domjudge/domjudge-contributor
     hostname: domjudge-contributor
     volumes:
-      - /sys/fs/cgroup:/sys/fs/cgroup:ro
+      - /sys/fs/cgroup:/sys/fs/cgroup
       - .:/domjudge:cached
       - /chroot
     links: