so, the issue is as I expected, nothing to do with prefect, but aptible's deploy environment.
TLDR:
• containers are only accessible through tcp load-balancers
• I tried to get clever and open a bunch of ports on the shared docker image, and on the ELB, and set the entrypoint for the containers to be subtly different per-container that we scaled with each worker listening on a distinct port
• i expected/hoped that the requests through the elb would either be routed directly to the right container (not a reasonable assumption), or they'd multicast to all the containers and whichever one was listening on that port would act and the others would ignore it
• Actually, aptible will hear a request on the elb at port e.g. 1234, and randomly pick from the containers behind the elb and pass through on that same port