LCOW: Implemented support for docker cp + build

This enables docker cp and ADD/COPY docker build support for LCOW.
Originally, the graphdriver.Get() interface returned a local path
to the container root filesystem. This does not work for LCOW, so
the Get() method now returns an interface that LCOW implements to
support copying to and from the container.

Signed-off-by: Akash Gupta <akagup@microsoft.com>
Upstream-commit: 7a7357dae1bcccb17e9b2d4c7c8f5c025fce56ca
Component: engine

components/engine/integration-cli/docker_cli_external_graphdriver_unix_test.go

@@ -198,12 +198,13 @@ func (s *DockerExternalGraphdriverSuite) setUpPlugin(c *check.C, name string, ex
 			return
 		}
 
+		// TODO @gupta-ak: Figure out what to do here.
 		dir, err := driver.Get(req.ID, req.MountLabel)
 		if err != nil {
 			respond(w, err)
 			return
 		}
-		respond(w, &graphDriverResponse{Dir: dir})
+		respond(w, &graphDriverResponse{Dir: dir.Path()})
 	})
 
 	mux.HandleFunc("/GraphDriver.Put", func(w http.ResponseWriter, r *http.Request) {