1. The idea behind the overlays is to "cache" and reuse as much as you can between images. If you have a long list of packages you install before you copy your app source to the container, you don't have to install the packages again and again every time you do a change to the source and rebuild.
2. You can point Docker to a file - use 'docker save' and 'docker load'.
2. You can point Docker to a file - use 'docker save' and 'docker load'.