Add comprehensive size reduction summary documentation

Co-authored-by: yutaro-sakamoto <[email protected]>

Author: Copilot

SIZE_REDUCTION_SUMMARY.md

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+# Docker Image Size Optimization Summary
+
+## Problem
+The original Docker images were unnecessarily large due to including all build dependencies, full JDK, and build artifacts in the final runtime image.
+
+## Solution
+Implemented multi-stage Docker builds with the following optimizations:
+
+### 1. Multi-Stage Architecture
+**Before (Single-stage):**
+```
+FROM almalinux:9
+├── Install all build tools (gcc, make, bison, flex, etc.)
+├── Install full JDK (java-11-openjdk-devel)
+├── Download and compile software
+└── Final image contains everything
+```
+
+**After (Multi-stage):**
+```
+# Build Stage
+FROM almalinux:9 AS builder
+├── Install build tools
+├── Download and compile software
+└── Stage files for copying
+
+# Runtime Stage  
+FROM almalinux:9
+├── Install minimal runtime (headless JRE only)
+├── Copy built files from builder
+└── Minimal final image
+```
+
+### 2. Dependency Optimization
+- **Build stage only**: gcc, make, bison, flex, automake, autoconf, diffutils, gettext, java-11-openjdk-devel
+- **Runtime stage only**: java-11-openjdk-headless (much smaller than full JDK)
+
+### 3. Cleanup Optimizations
+- Combined RUN commands to reduce Docker layers
+- Added `dnf clean all` to remove package manager caches
+- Added `rm -rf /var/cache/dnf/*` for additional cache cleanup
+- Source directories removed after compilation (`rm -rf /root/opensourcecobol4j-*`)
+
+### 4. Efficient File Management
+- Used `DESTDIR=/tmp/install` for staged installation
+- Only necessary runtime files copied with `COPY --from=builder /tmp/install/usr/ /usr/`
+- Build artifacts and intermediate files excluded from final image
+
+## Expected Size Reduction: 60-80%
+
+### Components Eliminated from Final Image:
+- **Build tools**: ~200-300MB (gcc, make, bison, flex, automake, etc.)
+- **Full JDK vs headless**: ~100-150MB (java-11-openjdk-devel → java-11-openjdk-headless)
+- **Package caches**: ~50-100MB (dnf cache, /var/cache/dnf/*)
+- **Source code & artifacts**: ~50-100MB (downloaded tarballs, build directories)
+
+### Total Estimated Reduction: ~400-650MB
+
+## Files Modified
+- `Dockerfile` - Multi-stage build for opensourcecobol4j + Open COBOL ESQL 4J
+- `utf8.Dockerfile` - Multi-stage build for opensourcecobol4j with UTF-8 support
+- `OPTIMIZATION.md` - Documentation of optimization techniques
+
+## Functionality Preserved
+Both optimized images maintain full functionality:
+- All required binaries and libraries included
+- Sample programs included
+- Environment variables and classpath correctly configured
+- Same user experience and capabilities as original images