A "truly noninteractive" Ubuntu-based Packer HCL template for dist-upgrade

template.pkr.hcl

build {
  # ...

  provisioner "shell" {
    inline = [
      "echo 'debconf debconf/frontend select Noninteractive' | debconf-set-selections",
      "sudo apt-get update",
      "sudo apt-get dist-upgrade -o Dpkg::Options::=--force-confold -o Dpkg::Options::=--force-confdef -y --allow-downgrades --allow-remove-essential --allow-change-held-packages",
      "sudo apt-get autoremove -y",
      "sudo apt-get clean",
    ]
  }
  
  # ...
}