Okay !
I understand best now, Thank you again for the clarification.
Yes with this network mode, mapping should just work, and that explains how yuou get the right IP.
Unfortunately for me I cant use this mode,
but good news for anyone stepping by with such issues : is is possible to use the good old JDBC_PING
see also here:
(requires a tweak in entrypoint to get prooper ip adress for ECSMETADATA before running kc.sg)