Add the ability to run crond inside the Docker container

- Add the crond init script for s6
- Add the RUN_CROND configuration variable to setup crond
- Crond will not be run by default (hence the `down` file in the service directory)
- `start.sh` check if RUN_CROND = "true" || "1" and remove this file to tell s6 to run the initscript
- Resolves #2597

docker/s6/crond/run

@@ -0,0 +1,9 @@
+#!/bin/sh
+# Crontabs are located by default in /var/spool/cron/crontabs/
+# The default configuration is also calling all the scripts in /etc/periodic/${period}
+
+if test -f ./setup; then
+    source ./setup
+fi
+
+exec gosu root /usr/sbin/crond -fS