Documentation for running it in docker should state about privileged option

[mareknovotny]

Description

https://github.com/spotify/docker-gc#running-as-a-docker-container describes how to run it in docker container, but without --privileged it fails. So that doc should be updated to have there
docker run --rm --privileged -v /var/run/docker.sock:/var/run/docker.sock -v /etc:/etc:ro spotify/docker-gc