There is a custom add-on functionality Martijn and Roberto just blogged about this week.
https://blog.martdj.nl/2024/10/10/building-custom-add-ons-for-your-domino-container-image/
https://www.robertoboccadoro.com/2024/10/10/upgrading-ontime-in-a-container/
This was the missing trigger for me to look into it again.
It's a quite new functionality which wasn't fully documented yet.
Documentation
I have added a new documentation mark down page-->https://opensource.hcltechsw.com/domino-container/concept_custom_addons/
This is a companion discussion topic for the original entry at https://blog.nashcom.de/nashcomblog.nsf/dx/domino-container-image-custom-add-on-support-enhancements.htm