Docker support for github-runner

I tried to add docker support for the [github-runner](https://github.com/GoogleCloudPlatform/cloud-run-samples/tree/main/github-runner) example but faced multiple issues.

I was able to start docker inside the container, but needed root permission for that, which in turn made the github runner fail.
I tried to run docker as a sidecar but got obscure docker errors then.

Do you have any suggestion on how to add docker support? A general guide on how to properly run "docker in docker" in cloud run could also help.