Add some documentation about relative WORKDIR values

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Upstream-commit: 69299f041f78f8a86bf810142e740bbd72fe4b1b
Component: engine

components/engine/docs/sources/reference/builder.rst

@@ -407,7 +407,16 @@ the image.
 The ``WORKDIR`` instruction sets the working directory for the ``RUN``, ``CMD`` and
 ``ENTRYPOINT``  Dockerfile commands that follow it.
 
-It can be used multiple times in the one Dockerfile.
+It can be used multiple times in the one Dockerfile.  If a relative path is
+provided, it will be relative to the path of the previous ``WORKDIR``
+instruction.  For example:
+
+    WORKDIR /a
+    WORKDIR b
+    WORKDIR c
+    RUN pwd
+
+The output of the final ``pwd`` command in this Dockerfile would be ``/a/b/c``.
 
 3.11 ONBUILD
 ------------