The Appian Release Notes for version 21.4 states the following on container-based installation of the Appian platform on a production environment.
Appian for Docker Containers has been replaced with a new implementation using Kubernetes. We are providing a Kubernetes Operator to manage Appian instances in your own non-production cluster using a single configuration.
This is a preview release of Appian on Kubernetes. Look for a production-ready version of Appian on Kubernetes in upcoming releases.
I am desparately waiting for news on that topic, and I do not find any further or more up-to-date information on that issue. Maybe you can help me to make the last sentence (emphasized by me) of the quote can be made less vague, please?