Support docker's entrypoint option

Docker has the '--entrypoint' which is used to overwrite the entry point of the docker image. It is useful to add support for this option in Zun.

Below is the docker command:

    $ docker run --help

    Usage: docker run [OPTIONS] IMAGE [COMMAND] [ARG...]

    Run a command in a new container

    Options:
          ...
          --entrypoint string Overwrite the default ENTRYPOINT of the image
          ...

This proposal is supporting this:

    $ zun run --entrypoint <...> <image> ...