Fix Dockerfile.image heredoc syntax to use printf instead

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

Dockerfile.image

@@ -102,14 +102,14 @@ RUN mkdir -p /output /mnt/esp /mnt/root && \
     \
     # Create GRUB configuration
     mkdir -p /mnt/esp/boot/grub && \
-    cat > /mnt/esp/boot/grub/grub.cfg << 'GRUB_EOF' && \
-set timeout=3
-set default=0
-
-menuentry "SparkOS" {
-    linux /boot/vmlinuz root=LABEL=SparkOS rw init=/sbin/init console=tty1 quiet
-}
-GRUB_EOF
+    printf '%s\n' \
+        'set timeout=3' \
+        'set default=0' \
+        '' \
+        'menuentry "SparkOS" {' \
+        '    linux /boot/vmlinuz root=LABEL=SparkOS rw init=/sbin/init console=tty1 quiet' \
+        '}' \
+        > /mnt/esp/boot/grub/grub.cfg && \
     \
     # Set up root filesystem
     echo "Setting up root filesystem..." && \
@@ -139,28 +139,28 @@ GRUB_EOF
     echo "spark:x:1000:" >> /mnt/root/etc/group && \
     \
     # Create README
-    cat > /mnt/root/README.txt << 'README_EOF' && \
-SparkOS UEFI-Bootable Image
-
-This is a UEFI-bootable disk image with:
-- GPT partition table
-- EFI System Partition (ESP) with FAT32 filesystem
-- GRUB UEFI bootloader
-- Linux kernel
-- SparkOS init system
-- Busybox utilities
-
-The image can be written to a USB drive and booted on UEFI systems:
-  sudo dd if=sparkos.img of=/dev/sdX bs=4M status=progress
-  sudo sync
-
-Boot options:
-- UEFI boot support (tested on x86_64 systems)
-- Automatic boot after 3 seconds
-- Console on tty1
-
-For more information, see: https://github.com/johndoe6345789/SparkOS
-README_EOF
+    printf '%s\n' \
+        'SparkOS UEFI-Bootable Image' \
+        '' \
+        'This is a UEFI-bootable disk image with:' \
+        '- GPT partition table' \
+        '- EFI System Partition (ESP) with FAT32 filesystem' \
+        '- GRUB UEFI bootloader' \
+        '- Linux kernel' \
+        '- SparkOS init system' \
+        '- Busybox utilities' \
+        '' \
+        'The image can be written to a USB drive and booted on UEFI systems:' \
+        '  sudo dd if=sparkos.img of=/dev/sdX bs=4M status=progress' \
+        '  sudo sync' \
+        '' \
+        'Boot options:' \
+        '- UEFI boot support (tested on x86_64 systems)' \
+        '- Automatic boot after 3 seconds' \
+        '- Console on tty1' \
+        '' \
+        'For more information, see: https://github.com/johndoe6345789/SparkOS' \
+        > /mnt/root/README.txt && \
     \
     # Sync and unmount
     echo "Finalizing image..." && \