Add support for the --entrypoint option of docker run

We can use `command` in `fig.yml` to override the default command, but this won't work if the docker image we are working with specifies a `ENTRYPOINT` for its containers.

In my case, the Dockerfile of the main service has specified something like `["gunicorn", "-c", "config.py"]` as its `ENTRYPOINT`, so I can't do things like `python manage.py syncdb` if I can't override the `ENTRYPOINT`.

Entrypoints can be overrdden by the `--entrypoint` option, this PR attempts to solve the problem by exposing this option in `fig`.