Personally, I think copying pkg folder from host to image is not a good idea because :

• it tightly couples the place from where you are building the image (your host) and the image itself. You could potentially have differences in resulting images depending on where you build the image, and that's probably what you don't want
• moreover, if you have automated builds (from CI for example), you're probably rebuilding the whole application from a clean environment each time, so there is no initial pkg folder to copy.

If you're familiar with Java world, I've already encountered that problem for images built with Maven. To speed up the build, some people are copying their local Maven repository (~/.m2) in the image to avoid redownloading artifacts. I don't particularly agree with that, since there is always a risk that their .m2 folder contains corrupted artifacts : therefore, the image built on their machine will be different than if it was built on a clean environment. It depends on whether you want to have consistent builds or quick builds (I prefer the former).

In conclusion, I think that building images from a clean environment, without depending on the host where the image is built, is a good practice. That's why I personally would not copy any files (except application source code!) inside the image.