The startup script idea mentioned here is probably your best bet for a stateless setup. You could essentially pack your user creation logic into a small script and run it via an initContainer or a post-start hook in your K8s deployment.
Another way to handle it without persistence is to use the jsreport CLI to import a prepared backup file (including the users) during the container's boot sequence. It keeps things clean and ensures your environment is identical every time it spins up.