Build Docker with Gradle

This is an experimental project for building docker by gradle. It's designed to verify Gogradle's capability, thus there are some notices:

• This is not a production-ready project.
• To satisfy Gogradle, a JDK 8+ should be pre-installed on your machine.
• $GOPATH is not necessary.
• vendor and vendor.conf are removed. Personally, I don't like vendor.
• Most logic code of Makefile has been rewrited in build.gradle. My goal is to replace make with gradle. You can run ./gradlew xxx instead of make xxx. For example, run ./gradlew binary to replace make binary.