cli-plugins: disable use of dial-stdio

Since docker#1654 so far we've had problems with it not working on Windows (npipe
lacked the `CloseRead` method) and problems with using tcp with tls (the tls
connection also lacks `CloseRead`). Both of these were workedaround in docker#1718
which added a nop `CloseRead` method.

However I am now seeing hangs (on Windows) where the `system dial-stdio`
subprocess is not exiting (I'm unsure why so far).

I think the 3rd problem found with this is an indication that `dial-stdio` is
not quite ready for wider use outside of its initial usecase (support for
`ssh://` URLs to connect to remote daemons).

This change simply disables the `dial-stdio` path for all plugins. However
rather than completely reverting 891b3d9 ("cli-plugins: use `docker system
dial-stdio` to call the daemon") I've just disabled the functionality at the
point of use and left in a trap door environment variable so that those who
want to experiment with this mode (and perhaps fully debug it) have an easier
path do doing so.

Signed-off-by: Ian Campbell <[email protected]>

cli-plugins/plugin/plugin.go

@@ -31,7 +31,11 @@ func runPlugin(dockerCli *command.DockerCli, plugin *cobra.Command, meta manager
 	PersistentPreRunE = func(_ *cobra.Command, _ []string) error {
 		var err error
 		persistentPreRunOnce.Do(func() {
-			err = tcmd.Initialize(withPluginClientConn(plugin.Name()))
+			var opts []command.InitializeOpt
+			if os.Getenv("DOCKER_CLI_PLUGIN_USE_DIAL_STDIO") != "" {
+				opts = append(opts, withPluginClientConn(plugin.Name()))
+			}
+			err = tcmd.Initialize(opts...)
 		})
 		return err
 	}