wip: Implement a scheduler to run recurring tasks (such as z-push-gabsync) (#123)

* add scheduler container for gabsync
Adds a general scheduler container to trigger tasks within containers.
Also adds gabsync to zpush image
* make scheduler dynamic
execute each cron job once at startup to see if they would succeed
* remove services scripts from core container (now is part of the scheduler)
* add "CRONDELAY" for tasks that should not be executed at startup
* add documentation
* clear out crontab at startup

zpush/start.sh

@@ -30,6 +30,9 @@ echo "Configuring Z-Push for use behind a reverse proxy"
 sed -e "s#define([\"']USE_CUSTOM_REMOTE_IP_HEADER[\"'],\s*false)#define('USE_CUSTOM_REMOTE_IP_HEADER', 'HTTP_X_FORWARDED_FOR')#" \
     -i /etc/z-push/z-push.conf.php
 
+sed -e "s#define([\"']USERNAME[\"'],\s*'')#define('USERNAME', 'SYSTEM')#" \
+    -i /etc/z-push/gabsync.conf.php
+
 echo "Ensure config ownership"
 chown -R www-data:www-data /run/sessions