add integration test for --workdir=/

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)

integration-cli/docker_cli_run_test.go

@@ -945,4 +945,18 @@ func TestModeHostname(t *testing.T) {
 	deleteAllContainers()
 
 	logDone("run - hostname and several network modes")
-}
+}
+
+func TestRootWorkdir(t *testing.T) {
+	s, _, err := cmd(t, "run", "--workdir", "/", "busybox", "pwd")
+	if err != nil {
+		t.Fatal(s, err)
+	}
+	if s != "/\n" {
+		t.Fatalf("pwd returned '%s' (expected /\\n)", s)
+	}
+
+	deleteAllContainers()
+
+	logDone("run - workdir /")
+}