Support formatting to show root process id in `docker ps`

[jswolf19]

This is a feature request to allow viewing the root process's id in the docker ps listing. It can currently be viewed by container using docker inspect -f '{{.State.Pid}}' <container-name_or_ID>

My immediate use case is trying to find the container responsible for zombie processes on my system. This use case is likely ameliorated with the --init flag, but (assuming this is not a difficult thing to do) I feel it is information about the container that could be useful in other situations, as well.