Add docker volumes janitor

This cleans up tmp and logrotates /var/log every 12 hours.

Note that this janitor is separate from the box janitor because they
run as different users.

Fixes #503

setup/container/systemd/docker_janitor.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Docker Volumes Janitor
+StopWhenUnneeded=true
+
+[Timer]
+# this activates it immediately
+OnBootSec=0
+OnCalendar=*-*-* 0,6,12,18:00:00
+Unit=docker_janitor.service
+