Creating a Root Filesystem for Ubuntu 20.04 ARM64 using Hashicorp Packer + Docker

README.md

From Docker Container to Bootable Disk Images but with Packer

Source based on Blog Post @iximiuz (Ivan Velichko)

Putting it together

1. Create the Filesystem Tarball with Kernel and SystemD

   packer build -only="filesystem.*" docker-ubuntu-focal-amd64.pkr.hcl
   

2. Create a Bootable Disk Image using:

   packer build -only="partition.*" docker-ubuntu-jammy-amd64.pkr.hcl
   

The linux.img will be available in the working directory

docker-ubuntu-jammy-amd64.pkr.hcl

source "docker" "fs-base" {
  image = "ubuntu:jammy"
  platform = "linux/amd64"
  # Tar ball of the container's filesystem will be available in working directory
  export_path = "./linux.tar"
}

source "docker" "partition-container" {
  image = "ubuntu:jammy"
  pull = false
  platform = "linux/amd64"
  cap_add = [ "SYS_ADMIN" ]
  device  = ["/dev/loop0"]
  volumes = { "/tmp/docker-os-amd64/": "/os"}
}

build {
  # Install Linux Kernel and SystemD packages for filesystem
  name = "filesystem"
  sources = [ "source.docker.fs-base" ]

  provisioner "shell" {
    inline = [
      "apt-get update",
      "apt-get install -y --no-install-recommends linux-image-virtual",
      "apt-get install -y --no-install-recommends systemd-sysv"
    ]
  }
}

build {
  # Create Blank Image, Create Boot Partition, Root Partition, Final Image in Container
  name = "partition"
  sources = [ "source.docker.partition-container" ]
  
  provisioner "shell" {
    scripts = [
      "./partition.sh"
    ]
  }
}

partition.sh

#!/bin/bash
set -e

echo "[Creating Blank Image]"

IMG_SIZE=$(expr 1024 \* 1024 \* 1024)

dd if=/dev/zero of=/os/linux.img bs=${IMG_SIZE} count=1

sfdisk /os/linux.img <<EOF
label: dos
label-id: 0x5d8b75fc
device: new.img
unit: sectors

linux.img1 : start=2048, size=2095104, type=83, bootable
EOF

echo "[Mounting Image and making it EXT4]"

OFFSET=$(expr 512 \* 2048)
losetup -o ${OFFSET} /dev/loop0 /os/linux.img
mkfs.ext4 /dev/loop0
mkdir /os/mnt
mount -t auto /dev/loop0 /os/mnt/

echo "[Mounting the Filesystem]"
tar -xvf /os/linux.tar -C /os/mnt/

echo "[Install EXTLinux Bootloader]"
apt-get install -y extlinux

echo "[Creating Boot Partition]"
extlinux --install /os/mnt/boot/

cat > /os/mnt/boot/syslinux.cfg <<EOF
DEFAULT linux
  SAY Now booting the kernel from SYSLINUX...
 LABEL linux
  KERNEL /vmlinuz
  APPEND ro root=/dev/sda1 initrd=/initrd.img
EOF

echo "[Creating Final Image]"
dd if=/usr/lib/syslinux/mbr/mbr.bin of=/os/linux.img bs=440 count=1 conv=notrunc
umount /os/mnt
losetup -D