Add ENTRYPOINT instruction to allow args and omit the cmd for docker run

This allows a user to set an ENTRYPOINT in the Dockerfile so when the resulting image is ran, the the client can pass args to *docker run* and not overwrite the ENTRYPOINT, unlike setting a default CMD.

References Issue #1008

Example:

```
FROM ubuntu
RUN apt-get update
ADD test /test
ENTRYPOINT /test
CMD -help
 
----------------------------
docker run 0f9cb9352c12
-n is required to echo your name.
docker run 0f9cb9352c12 -n="John"
Hello John
```