Add hook for downloading UPX binary in advance of building docker image

[smyrman]

It appears possible (although undocumentd) to run some scripts in advance/after a docker hub automatic build, as described here: http://windsock.io/automated-docker-image-builds-with-multiple-tags/.

Preferably we would use Git-LFS, but that appears to be difficult due to docker/hub-feedback#500.

As for now, we could at least attempt to change the code to download a tarball from the official release location: https://github.com/upx/upx/releases/tag/v3.92 and unpack it before the main build happens. This could be done via docker using a Docerfile.build that prepears binaries for the main contianer.