The use of pools.Copy avoids io.Copy's internal buffer allocation.
This commit replaces io.Copy with pools.Copy to avoid the allocation of
buffers in io.Copy.
Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com>
(cherry picked from commit 014095e6a07748d0e1ce2f759f5c4f4b3783e765)
Signed-off-by: Andrew Hsu <andrewhsu@docker.com>