This fix tries to address the issue raised in 29129 where
"--hostname" not working when running in "--net=host" for
`docker run`.
The fix fixes the issue by not resetting the `container.Config.Hostname`
if the `Hostname` has already been assigned through `--hostname`.
An integration test has been added to cover the changes.
This fix fixes 29129.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
(cherry picked from commit b0a7b0120f)
Signed-off-by: Victor Vieux <vieux@docker.com>