Add proper support for relative WORKDIR instructions

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Improve WORKDIR test to cover more edge cases

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Add some documentation about relative WORKDIR values

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4691 from tianon/relative-workdir

Add proper support for relative WORKDIR instructions