Bump rake-compiler/rake-compiler-dock-image from 1.5.0-mri-x86-mingw32 to 1.5.1-mri-x86-mingw32 in /docker

Bump rake-compiler/rake-compiler-dock-image from 1.5.0-mri-x86-mingw32 to 1.5.1-mri-x86-mingw32 in /docker #656