imagetools: respect --builder flag

The --builder flag was being ignored by imagetools because of pointer
problems. Essentially, because the root cmds aren't parsed immediately,
we need to pass a pointer to the builder string so that it can be
updated before the RunE function gets called.

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

commands/imagetools/root.go

@@ -6,7 +6,7 @@ import (
 )
 
 type RootOptions struct {
-	Builder string
+	Builder *string
 }
 
 func RootCmd(dockerCli command.Cli, opts RootOptions) *cobra.Command {