Add Docker-based disk image building for releases

Co-authored-by: johndoe6345789 <224850594+johndoe6345789@users.noreply.github.com>

scripts/build-image.sh

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+#!/bin/bash
+# Build SparkOS disk image using Docker
+# This script builds a .img file without requiring root on the host
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
+OUTPUT_DIR="$PROJECT_ROOT/release"
+
+echo "SparkOS Disk Image Builder (Docker)"
+echo "===================================="
+echo ""
+
+# Clean previous build
+if [ -f "$OUTPUT_DIR/sparkos.img.gz" ]; then
+    echo "Cleaning previous image..."
+    rm -f "$OUTPUT_DIR/sparkos.img.gz"
+fi
+
+mkdir -p "$OUTPUT_DIR"
+
+echo "Building disk image using Docker..."
+echo "This may take a few minutes..."
+echo ""
+
+# Build the image builder container
+cd "$PROJECT_ROOT"
+docker buildx build \
+    --file Dockerfile.image \
+    --target image-builder \
+    --tag sparkos:image-builder \
+    --load \
+    .
+
+# Extract the image
+echo ""
+echo "Extracting disk image..."
+CONTAINER_ID=$(docker create sparkos:image-builder)
+docker cp "$CONTAINER_ID:/output/sparkos.img.gz" "$OUTPUT_DIR/sparkos.img.gz"
+docker rm "$CONTAINER_ID" > /dev/null
+
+echo ""
+echo "SUCCESS! Disk image created:"
+echo "  Location: $OUTPUT_DIR/sparkos.img.gz"
+echo "  Size: $(du -h "$OUTPUT_DIR/sparkos.img.gz" | cut -f1)"
+echo ""
+
+# Show decompressed size
+echo "Decompressed size:"
+DECOMPRESSED_SIZE=$(gunzip -l "$OUTPUT_DIR/sparkos.img.gz" | tail -1 | awk '{print $2}')
+echo "  $(numfmt --to=iec-i --suffix=B $DECOMPRESSED_SIZE)"
+echo ""
+
+echo "To write to a USB drive:"
+echo "  gunzip $OUTPUT_DIR/sparkos.img.gz"
+echo "  sudo dd if=$OUTPUT_DIR/sparkos.img of=/dev/sdX bs=4M status=progress"
+echo ""
+echo "WARNING: Replace /dev/sdX with your actual USB device!"
+echo "         This will DESTROY all data on the target drive!"
+echo ""
+
+echo "To inspect the image:"
+echo "  gunzip -c $OUTPUT_DIR/sparkos.img.gz > /tmp/sparkos.img"
+echo "  sudo mount -o loop /tmp/sparkos.img /mnt"
+echo "  ls -la /mnt"
+echo "  sudo umount /mnt"