Apologies to all for the delay.
There was a recent change for security reasons that removed an unauthenticated fallback for docker pulls from docker hub. For anyone impacted by this, you can mitigate by moving the affected image to your Azure Container Registry and then adding DOCKER_REGISTRY_SERVER_USERNAME and DOCKER_REGISTRY_SERVER_PASSWORD to your app settings.
Adding no auth option is being worked on; however, there isn't an ETA on when it will be available at this time.