CI3 with docker nginx + php + mariadb. Session cookie in post

Hi there,
I am moving a CI3 app from a single server to run in 3 docker containers.
Using docker-compose with images: 

Code:

nginx:stable-alpine
php:8.1-fpm-alpine
mariadb:latest

With nginx set to the same config as previously, with the exception of the connection to PHP:

Code:

fastcgi_pass php:9000;

and PHP listening on :9000 (default within the image)
When I post to save a record, the controller also receives the session cookie in $_REQUEST.

This is obviously unwanted behaviour as it should be handled (and removed) before it gets to the application controller.
This does not happen in the monolithic setup.
Any ideas on what is causing it or how to fix it? 
Help, please   .
Gray