Include CLI plugins in help output on unknown flag.

Previously `docker --badopt` output would not include CLI plugins.

Fixes #1813

Signed-off-by: Ian Campbell <ijc@docker.com>
(cherry picked from commit 40a6cf7c47)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cmd/docker/docker.go

@@ -76,6 +76,9 @@ func setFlagErrorFunc(dockerCli *command.DockerCli, cmd *cobra.Command) {
 	// is called.
 	flagErrorFunc := cmd.FlagErrorFunc()
 	cmd.SetFlagErrorFunc(func(cmd *cobra.Command, err error) error {
+		if err := pluginmanager.AddPluginCommandStubs(dockerCli, cmd.Root()); err != nil {
+			return err
+		}
 		if err := isSupported(cmd, dockerCli); err != nil {
 			return err
 		}

e2e/cli-plugins/help_test.go

@@ -92,4 +92,18 @@ func TestGlobalHelp(t *testing.T) {
 		assert.Assert(t, is.Equal(res2.Stdout(), ""))
 		assert.Assert(t, is.Equal(res2.Stderr(), res.Stdout()))
 	})
+
+	t.Run("badopt", func(t *testing.T) {
+		// Running `docker --badopt` should also produce the
+		// same thing, give or take the leading error message
+		// and a trailing carriage return (due to main() using
+		// Println in the error case).
+		res2 := icmd.RunCmd(run("--badopt"))
+		res2.Assert(t, icmd.Expected{
+			ExitCode: 125,
+		})
+		assert.Assert(t, is.Equal(res2.Stdout(), ""))
+		exp := "unknown flag: --badopt\nSee 'docker --help'.\n" + res.Stdout() + "\n"
+		assert.Assert(t, is.Equal(res2.Stderr(), exp))
+	})
 }