fix: determinism on appended fat32 partition

Containerfile

@@ -65,7 +65,6 @@ COPY --from=cpio . /
 COPY --from=mtools . /
 COPY --from=xz . /
 COPY --from=grub . /
-COPY --from=util-linux . /
 
 FROM base as dev
 COPY --from=gcc . /
@@ -198,6 +197,8 @@ FROM build AS install
 ENV SOURCE_DATE_EPOCH=1
 RUN <<-EOF
     set -eux
+    dd if=/dev/zero bs=1M count=10 >> user.img
+    mformat -v user -i user.img -N 0 ::
     find iso -exec touch -hcd "@0" "{}" +
     xorrisofs \
         -output airgap.iso \
@@ -217,24 +218,8 @@ RUN <<-EOF
         -no-emul-boot \
         -isohybrid-gpt-basdat \
         -follow-links \
+        -append_partition 3 0xb user.img \
         iso/
-
-    # Increase ISO size by 512 MB to create space for the third partition
-    dd if=/dev/zero bs=1M count=512 >> airgap.iso
-
-    # Append a new partition that uses the additional space
-    echo ", +" | sfdisk --append airgap.iso
-
-    # Set the newly added third partition to FAT32
-    sfdisk --part-type airgap.iso 3 b
-
-    # Calculate the byte offset of the third partition
-    # This is done by finding the end of the first partition using fdisk,
-    # adding 1 sector, and multiplying by 512 (since each sector is 512 bytes).
-    OFFSET=$(fdisk -l airgap.iso | awk '/^airgap.iso1/ {print ($4 + 1) * 512}')
-
-    # Format the third partition as FAT32 and label it 'USER'
-    mformat -v USER -i airgap.iso@@$OFFSET ::
 EOF
 
 ## Minimal Autorun SD card image