Add an option to set the working directory.

This makes it possible to simply wrap a command inside a container. This makes
it easier to use a container as an unified build environment.

Examples:

~/workspace/docker
$ docker  run  -v `pwd`:`pwd` -w `pwd` -i -t  ubuntu ls
AUTHORS		 Makefile	archive.go	   changes.go	      docker
[...]


docker  run  -v `pwd`:`pwd` -w `pwd` -i -t  ubuntu pwd
/home/marco/workspace/docker
Upstream-commit: 687d27ab575778196ea646b6d3fa61b6c8e512b0
Component: engine

components/engine/docs/sources/api/docker_remote_api_v1.4.rst

@@ -129,7 +129,9 @@ Create a container
 		"Dns":null,
 		"Image":"base",
 		"Volumes":{},
-		"VolumesFrom":""
+		"VolumesFrom":"",
+		"WorkingDir":""
+
 	   }
 	   
 	**Example response**:
@@ -195,7 +197,9 @@ Inspect a container
 				"Dns": null,
 				"Image": "base",
 				"Volumes": {},
-				"VolumesFrom": ""
+				"VolumesFrom": "",
+				"WorkingDir":""
+
 			},
 			"State": {
 				"Running": false,
@@ -746,7 +750,8 @@ Inspect an image
 				,"Dns":null,
 				"Image":"base",
 				"Volumes":null,
-				"VolumesFrom":""
+				"VolumesFrom":"",
+				"WorkingDir":""
 			},
 		"Size": 6824592
 	   }

components/engine/docs/sources/commandline/command/run.rst

@@ -29,6 +29,7 @@
       -v=[]: Create a bind mount with: [host-dir]:[container-dir]:[rw|ro]. If "host-dir" is missing, then docker creates a new volume.
       -volumes-from="": Mount all volumes from the given container.
       -entrypoint="": Overwrite the default entrypoint set by the image.
+      -w="": Working directory inside the container
 
 
 Examples
@@ -62,3 +63,22 @@ cgroup controller. In other words, the container can then do almost
 everything that the host can do. This flag exists to allow special
 use-cases, like running Docker within Docker.
 
+.. code-block:: bash
+
+   docker  run -w /path/to/dir/ -i -t  ubuntu pwd
+
+The ``-w`` lets the command beeing executed inside directory given, 
+here /path/to/dir/. If the path does not exists it is created inside the 
+container.
+
+.. code-block:: bash
+
+   docker  run  -v `pwd`:`pwd` -w `pwd` -i -t  ubuntu pwd
+
+The ``-v`` flag mounts the current working directory into the container. 
+The ``-w`` lets the command beeing executed inside the current 
+working directory, by changeing into the directory to the value
+returned by ``pwd``. So this combination executes the command
+using the container, but inside the current working directory.
+
+