Add Docker build environment for Linux, Mac, Windows

[TwinFan]

Docker build environment

This adds a Docker environment to build Linux, Mac, and Windows similar to how LiveTraffic is built. I could successfully build all 3 targets but have only tested the resulting Mac binary.

Mainly adds the docker folder. See its README for some background on the Docker environment. lib/mingw-std-threads is necessary to teach MingW std::thread.

Other than that there were only some minor adaptations to CMakeLists.txt files needed.

I have changed binary output paths to proper X-Plane paths like XMidiCtrl/mac_x64/XMidiCtrl.xpl.

To build install Docker client on Linux, then

cd docker
make

MacOS Security Warning

Please bear in mind that the resulting MacOS binary is not signed, which will likely raise a security popup the first time you run X-Plane after the plugin installation. Something like

"XMidiCtrl.xpl" cannot be opened because the developer cannot be verified.

To solve this:

• Click Cancel. The plugin will not load this time.
• Now, open you macOS system settings and go to Security & Privacy, page General. You will see a note saying that XMidiCtrl was blocked.
• Click on the provided Allow Anyway button.
• Restart X-Plane. You will now see a different security warning:

  macOS cannot verify the developer of "XMidiCtrl.xpl". Are you sure you want to open it?.

• Click Open. X-Plane and the XMidiCtrl plugin will now load.

From now on, X-Plane and XMidiCtrl will start just fine. You will need to repeat this procedure only with the next plugin update.

[mauer]

Thank you so much for your work!!!

I really appreciate it. Using Docker and this whole cross platform compiling is still very new to me. So it's just wonderful to have a working environment I can go forward with.