How to setup docker env. to build github source code

why use docker env. to build banana pi source code allwinner tools needs ubuntu12.04 with ia32-libs … user maybe use ubuntu 12.04 or 14.04 and newer like 16.04, the docker env. the same, even other linux like fedora opensuse archlinux …, or other os like macos , windows … build BPI-Mx-bsp env. than mainline kernel & uboot or android 4.X or android 5.X / 6.X not the same.