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/cloudron.target

@@ -2,8 +2,8 @@
 Description=Cloudron Smart Cloud
 Documentation=https://cloudron.io/documentation.html
 StopWhenUnneeded=true
-Requires=box.service janitor.timer
-After=box.service janitor.timer
+Requires=box.service janitor.timer docker_janitor.timer
+After=box.service janitor.timer docker_janitor.timer
 # AllowIsolate=yes
 
 [Install]