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.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=Docker Volumes Janitor
+OnFailure=crashnotifier@%n.service
+
+[Service]
+Type=simple
+Restart=no
+ExecStart=/usr/sbin/docker_janitor
+KillMode=process
+User=root
+Group=root
+MemoryLimit=50M
+WatchdogSec=180
+