Add Docker test environment for gem install verification

[fs0414]

📋 Overview

Add a Docker environment to verify gem install works across multiple Ruby versions.

This allows us to detect build errors that only occur on specific Ruby versions (like Issue #70) locally before release.

🔧 Main Changes

🐳 Docker Environment

• Dockerfile.test: Dockerfile for verifying gem install and native extension builds

  • Installs Rust toolchain and LLVM/Clang
  • Automated flow: gem build → install → require verification → CLI test
  • Supports version switching via RUBY_VERSION ARG

• compose.test.yml: For building multiple versions simultaneously

  • Service definitions for Ruby 3.4 and 4.0
  • Run docker compose -f docker/compose.test.yml build for batch testing

📝 Scripts

• test-gem-install.sh: Test execution script
  • Tests all versions (3.4, 4.0) when run without arguments
  • Supports specific version (e.g., ./scripts/test-gem-install.sh 4.0)
  • Color-coded output for readability
  • Summary of pass/fail results

⚙️ Build Optimization

• .dockerignore: Optimizes build context
  • Excludes .nix-* cache directories (3GB+)
  • Reduces context transfer from 3GB to 1.6MB
  • Significantly faster build startup

🗂️ Changed Files

• Docker: docker/Dockerfile.test, docker/compose.test.yml
• Scripts: scripts/test-gem-install.sh
• Build: .dockerignore

🧪 Testing

How to Run Tests

# Test all versions (3.4, 4.0)
./scripts/test-gem-install.sh

# Test specific version only
./scripts/test-gem-install.sh 4.0

# Build with docker compose
docker compose -f docker/compose.test.yml build

Test Results

• Ruby 3.4.8: ✅ gem install success, require success
• Ruby 4.0.1: ✅ gem install success, require success

Verification Checklist

• gem install on Ruby 3.4
• gem install on Ruby 4.0
• require 'rfmt' works correctly
• .dockerignore speeds up builds

📦 Breaking Changes

None

Related Issue

Related to #70