Investigate cross-building Mixxx for aarch64 (perhaps even cross-building packages in general)

[fwcd]

See #12 for details

[fwcd]

The approach in https://archlinuxarm.org/forum/viewtopic.php?t=6163 seems to be worth investigating. We'd definitely need a cross-GCC and a sysroot (which we could provision from the first Docker stage) and then we'd have to make sure that the dependencies (of the package to be built) are installed to the sysroot for the target architecture.