Fix a regression introduced by 30c4637f03
which made the `docker run` command produce potentially truncated
stdout/stderr output.
Previous implementation stopped the content streaming as soon as the
container exited which would potentially truncate a long outputs.
This change fixes the issue by only canceling the IO stream immediately
if neither stdout nor stderr is attached.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
c27751fcfe