GitHub provides an agent that you can run. It polls GitHub for tasks and runs them locally. They also provide docker images and even a Kubernetes controller for auto scaling runners. Some people do it with lambda functions or plain ec2 launched on the fly.
If you meant to ask how to run the GitHub actions code without GitHub then the sibling comment would help for most tasks but has limitations.
Running the actions in a docker container is not ideal by any means. The authors of Github Actions do not have a stable API between the actions runner and whatever triggers them on their server side; you need to keep the docker container up-to-date every few months, it's not a "fire and forget" thing.
Yes. But act isn't entirely compatible unfortunately. To run the "real" thing, my latest attempt is here: https://github.com/efrecon/gh-runner-krunvm. It let you create ephemeral runners in microVMs. As soon as one runner has finished a job, a new one will be created. You can have several of those loops running in parallel on the same host.
