fix bug in FollowSymlinkInScope when link == root

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
add integration test for --workdir=/

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)
Merge pull request #6646 from tiborvass/fix-workdir

Fix --workdir=/