Extract embedded text from Dockerfiles and workflows to separate files

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

Dockerfile.image

@@ -138,29 +138,8 @@ RUN mkdir -p /output /mnt/esp /mnt/root && \
     echo "root:x:0:" > /mnt/root/etc/group && \
     echo "spark:x:1000:" >> /mnt/root/etc/group && \
     \
-    # Create README
-    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 && \
+    # Copy README to root partition
+    cp /build/config/image-readme.txt /mnt/root/README.txt && \
     \
     # Sync and unmount
     echo "Finalizing image..." && \