Allow using Docker actions when running within a Docker container

[jpb]

Add --allow-docker-in-docker flag to optionally disable the Container feature is not supported when runner is already running inside container constraint.

This resolves an issue described in #367 (comment).

I am successfully using this change on my self-hosted runner infrastructure which mounts the host's docker socket into the container where the actions runner executes.

[bryanmacfarlane]

@TingluoHuang

https://github.com/actions/runner/blob/master/docs/contribute.md#enhancements-and-feature-requests

[TingluoHuang]

@jpb thanks for your contribution, do you mind follow contribute.md to create a feature request issue and provide an ADR for the proposed changes.
the main concern I have is around volum mount and network setting, those might become different when the runner is running inside a container. We can discuss more in the Feature request issue. 😆

[jpb]

Yes – I have created a feature request: #406.

[DavidGOrtega]

@jpb I have forked the runner to do a quick implement of labels and support docker in docker. I dont need the flags so I just removed the exception. The final thing does not work.

  runs-on: [gpu]
    container:
      image: docker://davidgortega/gpu:runner
      options: --gpus "device=0" -v "/var/run/docker.sock":"/var/run/docker.sock"

    steps:
      - uses: actions/checkout@v2
      
      - name: run
        run: |
          nvidia-smi

Fails the checkout action

Did it work for you or do you have any advice before I get deeper?