logger.PutMessage, added in #28762 (v17.04.0-ce), clears msg.Source. So journald
and syslog were treating stderr messages as if they were stdout.
Signed-off-by: David Glasser <glasser@davidglasser.net>
(cherry picked from commit 917050c5728f2fb9958ccb3ab66a23766f741adc)
Signed-off-by: Andrew Hsu <andrewhsu@docker.com>