It looks like Pulumi does not support deploying Docker containers to Google App Engine Flex runtimes.
I dug a little deeper, and this seems like an upstream problem. There is an issue in the Terraform and Google Cloud repos to add support for this runtime into configuration settings:
Is this correct?
I just want to check to make sure, because if so that means that the only options for deploying containerized apps on GCP at the moment with Pulumi are through Cloud Run (which is great but does not support websocket functionality so is a no-go for apps that need live subscriptions) or as a k8s pod, which can be heavy for a one-off service or two.