Hello again,
I guess I had to write here to find what was not working, but maybe it was worth it because it looks like a bug.
It seems that whatever the path you mention in docker-compose.yml. Eg in my case:
ORS_CONFIG: ./openrouteservice/src/main/resources/ors-config.json
the configuration file that is used is the one in openrouteservice\docker\conf
So I modified the maximum_routes field in this file and now I can compute matrices 
Best,
Fabien