This is starting to make much more sense now.... the documentation for the docker container uses the example of having one container as the database server and what look like sidecar containers as clients, but it doesn't call out the fact that they aren't sidecar containers, nor that you can only have a single ZEO server running at a time. Given that new information, you're right, what I had in mind makes no sense.
Two more questions... in a docker scenario, do the clients need their own separate filestorage, and do those need to persist after the container has exited (i.e, persistent volumes)? I'm guessing the answer is "yes," but I wanted to be sure.
Second, do you have any ideas on why I'd be having trouble only in cases where the zeo server is using mounted volumes? If I start a separate zeo container, it works fine. If I mount a volume with a plone client container, it works fine. But if I start the separate zeo container with the volume mounted, it won't start.