Autoscale GitLab CI/CD runners and save 90% on EC2 costs

This post details the setup of an autoscaling cluster of GitLab CI/CD runners using docker-machine and AWS EC2 Spot Instances. It covers creating a spawner machine, installing gitlab-runner and docker-machine, configuring the `config.toml` for autoscaling with `docker+machine` executor, and leveraging `amazonec2-request-spot-instance=true` and `amazonec2-spot-price` for cost savings. It also discusses tuning `concurrent`, `limit`, and `IdleCount` parameters for optimal performance and cost efficiency.