Minimize Docker image size using multi-stage builds

[Copilot]

• Implement multi-stage Docker builds for both Dockerfile and utf8.Dockerfile
• Optimize Java runtime by using headless JRE instead of full JDK
• Add efficient cleanup and reduce Docker layers
• Use staged installation with DESTDIR for clean runtime image
• Remove unnecessary documentation files (OPTIMIZATION.md and SIZE_REDUCTION_SUMMARY.md)

Expected Results

• 60-80% size reduction (~400-650MB smaller images)
• Maintained functionality with all sample programs and libraries
• Cleaner repository without redundant documentation

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[yutaro-sakamoto]

@copilot
remove OPTIMIZATION.md and SIZE_REDUCTION_SUMMARY.md

[Copilot]

@copilot
remove OPTIMIZATION.md and SIZE_REDUCTION_SUMMARY.md

Both files have been removed as requested. Commit: 1bae142