Ubuntu18.04でAndroid版Mozcのapk作ろうと思ってるんだけど､
なんでDockerが必要なの?
C++だけで出来ないの？

Build Instructions
How to build Mozc in Docker: Android, NaCl, and Linux desktop builds.
How to build Mozc in OS X: OS X build.
How to build Mozc in Windows: Windows build.