install/uninstall: allow aliasing buildx to docker builder

Currently the user can type `docker buildx` to use this tool.
This patch allows the user to install buildx as a `docker builder` alias.

As an additional benefit, this allows the regular `docker build` to hook
into `buildx build`.

Note that the install and uninstall commands are currently hidden.

Signed-off-by: Tibor Vass <tibor@docker.com>

vendor/github.com/docker/cli/cli-plugins/plugin/plugin.go

@@ -40,10 +40,13 @@ func runPlugin(dockerCli *command.DockerCli, plugin *cobra.Command, meta manager
 		return err
 	}
 
-	cmd, _, err := tcmd.HandleGlobalFlags()
+	cmd, args, err := tcmd.HandleGlobalFlags()
 	if err != nil {
 		return err
 	}
+	// We've parsed global args already, so reset args to those
+	// which remain.
+	cmd.SetArgs(args)
 	return cmd.Execute()
 }