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`.
@dnephin Thanks for replying. Isn't this a function that return some information about an already running container? What do you mean by "this"?