Bugfix: only use io.Copy in hijack if attaching both stdout and stderr

Add regression tests to ensure issue is fixed.

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)
Upstream-commit: 1476f295aca20e1c35383c133219d54a5373183f
Component: engine

components/engine/api/client/hijack.go

@@ -88,7 +88,7 @@ func (cli *DockerCli) hijack(method, path string, setRawTerminal bool, in io.Rea
 			}()
 
 			// When TTY is ON, use regular copy
-			if setRawTerminal {
+			if setRawTerminal && stdout != nil {
 				_, err = io.Copy(stdout, br)
 			} else {
 				_, err = utils.StdCopy(stdout, stderr, br)