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.

Entrypoints can be overrdden by the `--entrypoint` option, this PR attempts to solve the problem by exposing this option in `fig`.
You should be able to do this with the `entrypoint` param to `client.create_container()`

Docs at https://github.com/docker/docker-py