RFC: Introduce a new build section on spec to use with `package_dir` if required

Inside the portage-tree with package per single package, there are a lot of layers and space required.
The use of the package_dir requires post-build steps that aren't really used in the next layers.

In general, in use cases like this, it's not needed that the files copied inside the package_dir directory that in a long time it duplicates the space required for the final image will be used.

Describe the solution you'd like
We could provide an epilogue section with steps that are executed in the images to get the package files but that isn't inside the pushed image.

epilogue:
- cp ${my_files} /${package_dir}/

It will add complexity but it reduces i/o operations that are very slow.