build script - download previously built and packed distributions

[shawnlaffan]

The build system starts from scratch each time a new docker instance is started, and hence all libs need to be rebuilt.

It would be useful if the build system downloaded those already-built packages from a server. This can be added to the perl scripts implemented in #17 to reduce the amount of rebuilding.

A config file can be used to specify the URL, defaulting to a path under https://strawberryperl.com/ that allows for gcc version and bitness differences (similar to https://strawberryperl.com/package/kmx/64_libs/ but flatter).

The download could also be done as part of the docker file.