Well, if you want private images it is normal that they charge you for it. What I advise you to do is to make the images public and mount the private part as a volume. This way you can upload the images wherever you want without worrying.
Another option if you want the resulting image to have something private is to create as much as you can in a public image and have a script that adds the private part as the last layer.
Or you can directly use Glowing Bear without installing anything.