Upgrade to JUCE 6.1.5 + fluidsynth 2.2.5 + make Docker cross-compile parallel & ARM-native

[Birch-san]

Making a new Dockerfile for the build:

• inheriting from an ARM-native image (because my computer is ARM and compilation was super slow)
• making use of multi-stage builds and Docker buildkit
  • for concurrency (because builds are slow, and Docker for Mac's VM was never using more than 1 core for the build)
  • to make it less sensitive to layer ordering (invalidate layer cache in fewer situations)

Upgrading fluidsynth to 2.2.5, to fix #22 (comment).
Upgrading from my fork of JUCE 6.1.2 to mainline JUCE 6.1.5; the MinGW-w64 build has had some attention, so @reuk recommends to try it out, to see whether the following issues are fixed:

• Linking of VST3 plugin fails on Clang + MinGW due to CheckSettingMacros undefining JucePlugin_Build_VST3 juce-framework/JUCE#993
• Compilation fails on (Windows) Clang 13 due to inline ASM juce-framework/JUCE#986
• Compilation failure on Windows + Clang 13 on JUCE 6.1.4, expanding __uuidof(x) macro juce-framework/JUCE#985

Unfortunately, the changes mean that MinGW-w64 builds now depend on the Windows SDK, for example for headers like UIAutomation.h:
https://github.com/juce-framework/JUCE/blob/53b04877c6ebc7ef3cb42e84cb11a48e0cf809b5/modules/juce_gui_basics/juce_gui_basics.cpp#L70

I've acquired the Windows SDK via xwin. So I have the headers and libraries I need.

But llvm-mingw is failing to compile the code contained in the Windows SDK headers:
https://gist.github.com/Birch-san/d54e6e64be4e954b59a5156f76472a0a

Maybe it's fundamentally unsupported for clang to include the Windows SDK headers?

• https://stackoverflow.com/a/3403307/5257399
• Headers generated by MIDL compiler (such as those in the Windows SDK) aren't supported by MinGW (admittedly doesn't mention Clang)

Maybe we can use the best-effort headers generated by WIDL compiler instead?
https://github.com/slimsag/win32metadata/tree/master/generation/WinSDK/RecompiledIdlHeaders

I'm currently working on a minimal repro here (which seems to be failing on JUCE rather than Windows SDK):
https://github.com/Birch-san/juce-repro
The current error I'm getting in the minimal repro, is a uuidof() error, so looks related to juce-framework/JUCE#985.

Update: Okay, looks like there was an easier way to get the Windows SDK headers. MinGW comes with some already; I just needed to deal with the case-sensitive filename.