[Feature request] Allow static builds

[doragasu]

This is awesome, thanks for the work creating this Docker image.

Could you please add also support for static builds? For projects compatible with LGPL, having the option to build binaries statically is great. I do it regularly in my Arch machine, and IIRC the only thing I had to install was mingw-w64-qt5-base-static, then add to the .pro file:

CONFIG+=static
CONFIG+=no_smart_library_merge
QTPLUGIN+=qwindows

[mdimura]

Yes, I guess I could add an all-static version of the image