For instance/node level autoscaling (which is closer to what you need), I would recommend using the autoscaling features provided by AWS/Google Cloud.
It would have to be integrated with Kubernetes though -- when we push a new docker container, the container would need to be updated on any new machines created. We'll look into GCP's autoscale solution.
Even if you don't need autoscaling, I'd suggest still using autoscaling groups and setting it to a fixed number of instances, so that instances will automatically get restarted if they go down.
As for image management, it would depend on how you would like to propagate new images. With a private docker registry, you could potentially point each new instance to the registry and take care of propagating new images. I favor this approach since it keeps everything separate and easier to manage.