build: fix missing "docker" driver name in build progress

This was missing, since the driver property can only be fully populated
after loading nodes from disk. So we add logic to load the nodes, and
check for an error, which ensures that the "docker" driver is always
correctly present in the progress description.

Signed-off-by: Justin Chadwell <me@jedevc.com>

commands/build.go

@@ -233,6 +233,10 @@ func runBuild(dockerCli command.Cli, options buildOptions) (err error) {
 	if err != nil {
 		return err
 	}
+	_, err = b.LoadNodes(ctx, false)
+	if err != nil {
+		return err
+	}
 
 	ctx2, cancel := context.WithCancel(context.TODO())
 	defer cancel()