How to make "container:" in jobs work with DinD?

[zxti]

I'm able to run ARC with docker fine using containerMode: dind.

But I'd also like to enable my users to specify container: in Actions workflow jobs, and start docker builds etc. from there.

My understanding is that specifying container: on an Actions workflow job will cause the actions-runner container in the pod to spin up another docker container. But in here, there's no socket:

docker: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?.

How would one make this work? Is this possible?