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

setup.sh

@@ -65,7 +65,7 @@ docker_tag_search () {
 }
 
 echo "Creating individual env files for containers (if they do not exist already)"
-for dockerenv in ldap password-self-service mail db kopano_ssl kopano_server kopano_webapp kopano_zpush kopano_grapi kopano_kapi kopano_dagent kopano_spooler kopano_gateway kopano_ical kopano_monitor kopano_search kopano_konnect kopano_kwmserver kopano_meet; do
+for dockerenv in ldap password-self-service mail db kopano_ssl kopano_server kopano_webapp kopano_zpush kopano_grapi kopano_kapi kopano_dagent kopano_spooler kopano_gateway kopano_ical kopano_monitor kopano_scheduler kopano_search kopano_konnect kopano_kwmserver kopano_meet; do
 	touch ./"$dockerenv".env
 done