Start build on a windows ec2 with gitlab runner

New to gitlab CI. I have a custom windows ami (with required pre-installed software) where I would like to run the build. One of my options is to keep this instance always ON and install the runner on it, but that seems like a waste of compute. Can I start the instance from gitlab CI? I realize that the runner still needs to be running somewhere. I referred to the AWS autoscaling doc available on gitlab, but it highlights how to autoscale with docker.
I cannot use the docker setup, kind of tied to the custom image.

How do I achieve this workflow?