Add a little "paperclip" suggestion when attempting to run "dockerd" in the non-dind image variants

[tianon]

Fixes #61

Here's the example output:

$ docker run -it --rm docker:17.06-rc dockerd

📎 Hey there!  It looks like you're trying to run a Docker daemon.

   You probably should use the "dind" image variant instead, something like:

     docker run --privileged --name some-overlay-docker -d docker:stable-dind --storage-driver=overlay

   See https://hub.docker.com/_/docker/ for more documentation and usage examples.

WARN[0000] could not change group /var/run/docker.sock to docker: group docker not found
...

[tianon]

Worth noting (IMO) is that folks who know what they're doing can trivially get around this entire thing by using --entrypoint dockerd instead. 👍