Creating bootable images

This document describes the requirements to create standard container images that can be used for Elemental deployments

You can find the examples below in the examples folder.

From standard images

Besides using the elemental-toolkit toolchain, it’s possible to create standard container images which are consumable by the vanilla Elemental images (ISO, Cloud Images, etc.) during the upgrade and deploy phase.

An example of a Dockerfile image can be:

# run `make build` to build local/elemental-toolkit image
ARG TOOLKIT_REPO=local/elemental-toolkit
ARG VERSION=latest
ARG OS_IMAGE=registry.opensuse.org/opensuse/tumbleweed
ARG OS_VERSION=latest

FROM ${TOOLKIT_REPO}:${VERSION} AS toolkit

# OS base image of our choice
FROM ${OS_IMAGE}:${OS_VERSION} AS os
ARG REPO
ARG VERSION
ENV REPO=${REPO}
ENV VERSION=${VERSION}

# Workaround for RISC-V, specific kernel might be needed for some boards
ARG ADD_REPO
ENV ADD_REPO=${ADD_REPO}

# Install kernel, systemd, dracut, grub2 and other required tools
RUN ARCH=$(uname -m); \
    zypper --non-interactive removerepo repo-update || true; \
    if [[ -n "${ADD_REPO}" ]]; then \
      zypper --non-interactive addrepo --enable --refresh ${ADD_REPO} added-repo; \
    fi; \
    if [[ "${ARCH}" != "riscv64" ]]; then \
      ADD_PKGS+=" shim"; \
      [[ "${ARCH}" == "aarch64" ]] && ARCH="arm64"; \
    fi; \
    zypper --non-interactive --gpg-auto-import-keys install --no-recommends -- \
      kernel-default \
      device-mapper \
      dracut \
      grub2 \
      grub2-${ARCH}-efi \
      haveged \
      systemd \
      NetworkManager \
      openssh-server \
      openssh-clients \
      timezone \
      parted \
      e2fsprogs \
      dosfstools \
      mtools \
      xorriso \
      findutils \
      gptfdisk \
      rsync \
      squashfs \
      lvm2 \
      tar \
      gzip \
      vim \
      which \
      less \
      sudo \
      curl \
      sed \
      iproute2 \
      podman \
      audit \
      patterns-microos-selinux \
      btrfsprogs \
      btrfsmaintenance \
      snapper \
      xterm-resize \
      ${ADD_PKGS} && \
    zypper clean --all

# Just add the elemental cli
COPY --from=toolkit /usr/bin/elemental /usr/bin/elemental

# Enable essential services
RUN systemctl enable NetworkManager.service && \
    systemctl enable sshd.service

# Workaround to make sure there are no pending sysusers to be created (boo#1231244)
RUN systemd-sysusers

# This is for automatic testing purposes, do not do this in production.
RUN echo "PermitRootLogin yes" > /etc/ssh/sshd_config.d/rootlogin.conf

# SELinux in enforce mode
#RUN sed -i "s|SELINUX=.*|SELINUX=enforcing|g" /etc/selinux/config

# Add default snapshotter setup
ADD snapshotter.yaml /etc/elemental/config.d/snapshotter.yaml

# Generate initrd with required elemental services
RUN elemental --debug init --force

# Update os-release file with some metadata
RUN echo IMAGE_REPO=\"${REPO}\"         >> /etc/os-release && \
    echo IMAGE_TAG=\"${VERSION}\"           >> /etc/os-release && \
    echo IMAGE=\"${REPO}:${VERSION}\" >> /etc/os-release && \
    echo TIMESTAMP="`date +'%Y%m%d%H%M%S'`" >> /etc/os-release && \
    echo GRUB_ENTRY_NAME=\"Elemental\" >> /etc/os-release

# Good for validation after the build
CMD ["/bin/bash"]
Complete source code: https://github.com/rancher/elemental-toolkit/blob/main/examples/green/Dockerfile

We can just run docker to build the image with

docker build -t $IMAGE .

The important piece is that an image needs to ship at least:

grub2
systemd
kernel
dracut

And then extract the configuration for the system using the elemental init-command.

Customizations

All the method above imply that the image generated will be the booting one, there are however several configuration entrypoint that you should keep in mind while building the image:

  • Everything under /system/oem will be loaded during the various stage (boot, network, initramfs). You can check here for the Elemental defaults. See 00_rootfs.yaml to customize the booting layout.
  • /etc/cos/bootargs.cfg contains the booting options required to boot the image with GRUB

Last modified August 31, 2023: More documentation work (#1819) (2360815df)