Ignore unknown arguments on the top-level command.

This allows passing argument to plugins, otherwise they are caught by the parse
loop, since cobra does not know about each plugin at this stage (to avoid
having to always scan for all plugins) this means that e.g. `docker plugin
--foo` would accumulate `plugin` as an arg to the `docker` command, then choke
on the unknown `--foo`.

This allows unknown global args only, unknown arguments on subcommands (e.g.
`docker ps --foo`) are still correctly caught.

Add an e2e test covering this case.

Signed-off-by: Ian Campbell <ijc@docker.com>

e2e/cli-plugins/run_test.go

@@ -144,6 +144,18 @@ func TestRunGoodSubcommand(t *testing.T) {
 	})
 }
 
+// TestRunGoodArgument ensures correct behaviour when running a valid plugin with an `--argument`.
+func TestRunGoodArgument(t *testing.T) {
+	run, cleanup := prepare(t)
+	defer cleanup()
+
+	res := icmd.RunCmd(run("helloworld", "--who", "Cleveland"))
+	res.Assert(t, icmd.Expected{
+		ExitCode: 0,
+		Out:      "Hello Cleveland!",
+	})
+}
+
 // TestHelpGoodSubcommand ensures correct behaviour when invoking help on a
 // valid plugin subcommand. A global argument is included to ensure it does not
 // interfere.

e2e/cli-plugins/testdata/docker-help-helloworld.golden

@@ -1,8 +1,11 @@
 
-Usage:	docker helloworld COMMAND
+Usage:	docker helloworld [OPTIONS] COMMAND
 
 A basic Hello World plugin for tests
 
+Options:
+      --who string   Who are we addressing? (default "World")
+
 Commands:
   apiversion  Print the API version of the server
   goodbye     Say Goodbye instead of Hello