Well, I finally got it working with 172.17.0.1 and this script for the task scheduler
sleep(60) && /volume1/homes/admin/simple-synoindex-server/synoindex-server &
but there is a small limitation due to this bug :
New file is indexed only if a directory already exists.
It is well documented here : Possible Synology indexer notification bug - #9 by Igor
Do you know if this issue will be adressed in v3 ?