runner --driver=github Docker-in-Docker

Docker-in-Docker (probably) doesn't work with GitHub?

(P.S. we recommend `--docker-volumes=/var/run/docker.sock:/var/run/docker.sock` for GitLab)

```yml
jobs:
deploy:
steps:
- run: cml runner --cloud=aws --labels=cml-aws
train:
needs: deploy
runs-on: [self-hosted, cml-aws]
container:
image: docker://
steps:
- uses: [self-hosted, cml-aws]
- run: docker run ...
```

1. Have we tried using `docker:dind` for ``?
- Do we need `container.volumes: /var/run/docker.sock:/var/run/docker.sock`?
2. Have we confirmed *not* using a `container:` works (i.e. Docker-on-baremetal)?