Executing a GitHub runner in a container is a no-brainer, but it's still easy to overload the _host_ with too many jobs/containers, so managing resources is always up to the operator.
I understand why GitHub has this constraint - to avoid clashes between jobs. If multiple jobs (from the same repository) are executing simultaneously within the same stateful environment, they are more likely to clash over shared resources (/tmp, cleanup tasks, database names, etc.). However, even if my jobs are clean and idempotent, GitHub is nudging me to think about runners as "VMs" rather than "containers" (because often CI jobs involve their own containers, and docker-in-docker is a pain), and "self-hosting a bunch of CI VMs" becomes expensive rather quick.
well, GHA supports running actions in containers, which is the best way to control the environment your CI/CD runs in, so putting your runner in a container won't work for everyone.
but, if it does work for someone, doing what you've done will give a much better experience to their developers.
my employer uses single-user VMs for it's runners; it works well, but sometimes actions invocation is high and it can take a few minutes for a runner to come around to taking my job. that would be much less of a problem with dockerized runners.
I understand why GitHub has this constraint - to avoid clashes between jobs. If multiple jobs (from the same repository) are executing simultaneously within the same stateful environment, they are more likely to clash over shared resources (/tmp, cleanup tasks, database names, etc.). However, even if my jobs are clean and idempotent, GitHub is nudging me to think about runners as "VMs" rather than "containers" (because often CI jobs involve their own containers, and docker-in-docker is a pain), and "self-hosting a bunch of CI VMs" becomes expensive rather quick.