Fix to ensure that ConfigMaps volume does create file(s) in container

Document the fix proposed originally in:
kubernetes/kubernetes#23392 (comment)

Tested with:
Client Version: version.Info{Major:"1", Minor:"2", GitVersion:"v1.2.4", GitCommit:"3eed1e3be6848b877ff80a93da3785d9034d0a4f", GitTreeState:"clean"}
Server Version: version.Info{Major:"1", Minor:"2", GitVersion:"v1.2.4", GitCommit:"3eed1e3be6848b877ff80a93da3785d9034d0a4f", GitTreeState:"clean"}

docs/getting-started-guides/docker.md

@@ -34,7 +34,7 @@ docker run -d \
     --volume=/:/rootfs:ro \
     --volume=/sys:/sys:ro \
     --volume=/var/lib/docker/:/var/lib/docker:rw \
-    --volume=/var/lib/kubelet/:/var/lib/kubelet:rw \
+    --volume=/var/lib/kubelet/:/var/lib/kubelet:rw,shared \
     --volume=/var/run:/var/run:rw \
     --net=host \
     --pid=host \