duneci-runner: add anonymous volume for /tmp

gitlab-runner now writes traces to the file system and thus needs to
be able to write somewhere.

bin/duneci-runner

@@ -7,6 +7,7 @@ options=(
   -d --name gitlab-runner --restart always
   --read-only
   --stop-signal SIGQUIT
+  -v /tmp
   -v /var/run/docker.sock:/var/run/docker.sock
   -v /srv/gitlab-runner/config:/etc/gitlab-runner
 )