Thanks for fast reply and guiding me to the discussion link,, I posted my comment there.
To answer your question:
If I would be on a green-field I would just go for /var/lib/dockerdata..
But unfortunately I am on a brown field and have to lock how to stay backward compatible.
I would prefer an immutable directory creation option in the ignition config that allows
me to create a folder under root (with immutable = true) that is just used as mount dir..