Hi Jeroen,
no experience with Docker/Kubernetes, but if you can open a SSH shell to the actual server you should be able to use the m2ee utility to adjust loglevels (see also https://forum.mendix.com/link/questions/2259)
In addition, I came accross a blog post indicating that you can send an HTTP request to the admin port of the runtime in order to adjust loglevels (http://www.mxblog.eu/p/post/2533274790397906). Not sure if this still works in recent mendix versions though (the official documentation at https://docs.mendix.com/refguide/monitoring-mendix-runtime does not mention the set_log_level as an allowed option)
Hope this helps!
Try:
environment:
- LOGGING_CONFIG={"Core":"TRACE"}