Why I did it
It's possible to speed up some parts of a build using parallel compression/decompression.
This is especially important for build_debian.sh.
How I did it
pigz is a parallel implementation of gzip: https://zlib.net/pigz/
Some programs like docker and mkinitramfs can automatically detect and use it instead of gzip.
For tar we need to select it directly.
To enable this feature you need to set GZ_COMPRESS_PROGRAM=pigz
