add nsenter and docker-enter

help to debug docker containers by providing an easy
way to run a cmd (eg bash) inside an already running container.
docker-enter is a script that uses nsenter to do this.  it should
be run as root (or via sudo).  these come from
https://github.com/jpetazzo/nsenter
docker-enter: improve to use nsenter from path, if found, else local dir

note that when run with sudo it looks in the path for root, not your
current path.

this just makes it easier to use without installing nsenter into
/usr/local/bin, for example