ExpandEnv for ENTRYPOINT and CMD in runtime

The issues are discussed here,
https://groups.google.com/forum/#!topic/docker-user/tnscIyLxXVA
and here,
https://groups.google.com/forum/#!topic/docker-user/Hrt5SQgiB5E

> Feature: Evaluate Env variables before a command (from Cmd or Entrypoint) is invoked inside a container.

ENTRYPOINT ["./executable", "$VAR"]
CMD ["--host=$VAR"]

`$VAR` gets evaluated to it's corresponding value in the container's `env` during `runtime`. 

With this we can do things like, 

    docker run imagename ./executable --host \$VAR

with the `$` escaped in the host, and it will be executed in the container's environment. 

Tried tracing how a command is invoked within the container. But failed somewhere and ended up modifying the code responsible for building - buildfile.go - the function `buildCmdFromJson()` and included `os.ExpandEnv(args)` which converts the `env` vars in the string `args` to their corresponding values.