monitor: refactor how reload works

The build now happens in a loop and the monitor is run after every
build. The monitor can return `ErrReload` to signal to the main thread
that it should reload the build result.

This will be used in the future to move the monitor into a callback
rather than as a separate existence. It allows the monitor to not
control the build itself which now makes it possible to completely
remove the controller.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

commands/debug/root.go

@@ -53,7 +53,7 @@ func RootCmd(dockerCli command.Cli, children ...DebuggableCmd) *cobra.Command {
 				return errors.Errorf("failed to configure terminal: %v", err)
 			}
 
-			_, err = monitor.RunMonitor(ctx, "", nil, &controllerapi.InvokeConfig{
+			err = monitor.RunMonitor(ctx, &controllerapi.InvokeConfig{
 				Tty: true,
 			}, c, dockerCli.In(), os.Stdout, os.Stderr, printer)
 			con.Reset()