Add Docker-based build environment

[dajiaohuang]

Summary

• add a reusable Docker builder with the project's Java, Gradle, Android SDK, NDK, and CMake toolchain
• verify the command-line-tools and Gradle downloads with SHA-256 checksums
• document how to build the image and produce the Java and native artifacts from a mounted checkout

Scope

This keeps the existing local-toolchain workflow unchanged. The image is built locally; this PR does not add a registry-publishing service, credentials, or CI workflow.

Validation

• docker build --check . (no warnings)
• docker build -t fastbot-android-builder .
• docker run --rm -v "${PWD}:/workspace" fastbot-android-builder
  • Gradle completed 55 tasks successfully
  • generated monkeyq.jar
  • generated libfastbot_native.so for armeabi-v7a, arm64-v8a, x86, and x86_64

Closes #247