Trying to run "dockerd" in the non-dind image is likely to fail

[tianon]

As reported by @stevvooe over in docker-library/official-images#2654, it's a little bit non-obvious that the dind variants are the only place we have the dependencies for properly running Docker-in-Docker (and that the non-dind tags are simply the client).

He suggested having docker-entrypoint.sh (not dockerd-entrypoint.sh) print a warning if $1 is dockerd, something like:

📎 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 ...

[tianon]

Got something up in #64. 👍