I just faced a strange CPU 100% issue with docker images of plone 5.2.8 and 5.2.9 (5.2.7 is safe).
The problem raises when you start a zeoclient behind an already started haproxy daemon. If you start a zeoclient and then the haproxy, the cpu behaves normally.
The very weird thing is that starting zeoclient after haproxy yields CPU 100% BUT shutting down haproxy has no effect and the cpu still remains stuck on 100% of load (the process that loads the CPU is the /plone/instance/parts/instance/bin/interpreter one).
I have reproduced the problem using official vanilla plone docker images and eeacms/haproxy on two different linux distributions (different docker/docker-compose versions) so I'm pretty sure it's a deterministic issue.
Starting a Plone5.2.9/5.2.8 in a local buildout behind the same eeacms/haproxy docker daemon is free of this issue so far.