Failure to get the container id

[adarnimrod]

With #1888 in place, there's a regression when running inside a container. There's an assumption in https://github.com/pre-commit/pre-commit/blob/master/pre_commit/languages/docker.py#L32 that the hostname is the container ID which is not always the case (it's easy enough to set a different hostname with docker run --hostname foo). It causes the docker inspect command that follows to fail. A more reliable way to get the container id is from /proc/1/cpuset or from the first line in /proc/1/cgroup which is already checked in _is_in_docker.

Thanks @asottile and @okainov for your work on #1387 and pre-commit in general.