Fix truncated output in `fig run` - resolves #658

Signed-off-by: Michael Malet <MichaelMa@square-enix.com>
We're using dockerpty because it handles the streaming of the container's stdin/out/err to the terminal's stdin/out/err, as well as setting/unsetting raw mode on the terminal and resizing the pseudo-tty. We used to do all that in fig and it was hacky and gross.

As @dnephin said, we need to pass the right options (`ports=None, one_off=True`) to dockerpty now that we're not starting the container ourselves.