allow passing on of docker socket

[maxheld83]

currently:

Running docker run --volume '/var/run/docker.sock:/var/run/docker.sock' \
  'alpine:3.10.1' sh -c 'exit 1'
docker: Error response from daemon: This Docker operation is forbidden by GitHub Actions, you can find documentation at https://developer.github.com/actions/.
See 'docker run --help'.

That's not great, because it disallows a elegant way to run docker in docker as per #271

[maxheld83]

send email to support.

[maxheld83]

notably, the "official" Docker GitHub action does not appear to pass on the upstream socket, but just really runs docker in docker, though that is discouraged.