With current implementation there was a possibility
that /start responds quicker than /attach, meaning that
some output would be clipped.
Fixed so the implementation matches with `docker run`.
This also fixes the flaky test results for TestCreateEchoStdout.
Signed-off-by: Tõnis Tiigi <tonistiigi@gmail.com> (github: tonistiigi)
Upstream-commit: 79a77a396e003d0f5827c48d1b179f87c1542311
Component: engine