GitLab Runner: Keep cache volumes around between restarts.

Change-Id: Id601524671db43a32833beea841387cd26410be9
diff --git a/gitlab-system/gitlab-runner/default.nix b/gitlab-system/gitlab-runner/default.nix
index e98d43f..313accf 100644
--- a/gitlab-system/gitlab-runner/default.nix
+++ b/gitlab-system/gitlab-runner/default.nix
@@ -37,7 +37,7 @@
       sleep 10s
 
       ${pkgs.docker}/bin/docker kill $(${pkgs.docker}/bin/docker ps -a -q) || :
-      ${pkgs.docker}/bin/docker system prune --volumes --force || :
+      ${pkgs.docker}/bin/docker system prune --force || :
 
       exec ${pkgs.gitlab-runner}/bin/gitlab-runner run
     '';