machine registration: Minify the image and add RISCV64 support!
The net result is an uncompressed tarball size of 40 MB vs the current 129 MB, resulting in faster downloads and lower disk/network usage \o/.
As a bonus, generating new versions of the machine registration container should now take a couple of minutes rather than ~40 minutes like currently, and it will allow us to add support for ... RISCV64!