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ref-manual/classes.rst: document the image-container class

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Add documentation for the image-container class, which is a simple class
to generate an image suitable for creating a container.

This answers in part to questions asked in [YOCTO #14368].

It also adds documentation for IMAGE_CONTAINER_NO_DUMMY, which was added
in OE-Core with commit f0645e172bb8 ("image-container.bbclass: Error if
not using linux-dummy").

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
(From yocto-docs rev: 85fb6e4a964ea2dea9c3083ba2c4ceb336f34b1a)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 6ce00e5875eb3469fefd55cc22acaaeaf620053a)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
This commit is contained in:
Antonin Godard
2025-12-17 10:16:30 +01:00
committed by Richard Purdie
parent 9b6d0d6e5a
commit 71615e330c
2 changed files with 65 additions and 0 deletions
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@@ -1248,6 +1248,53 @@ The :ref:`ref-classes-image_types` class also handles conversion and compression
:term:`IMAGE_FSTYPES`. This would also be similar for Virtual Box Virtual Disk
Image ("vdi") and QEMU Copy On Write Version 2 ("qcow2") images.
.. _ref-classes-image-container:
``image-container``
===================
The :ref:`ref-classes-image-container` class is automatically inherited in
:doc:`image </ref-manual/images>` recipes that have the ``container`` image type
in :term:`IMAGE_FSTYPES`. It provides relevant settings to generate an image
ready for use with an :wikipedia:`OCI <Open_Container_Initiative>`-compliant
container management tool, such as :wikipedia:`Podman <Podman>` or
:wikipedia:`Docker <Docker_(software)>`.
.. note::
This class neither builds nor installs container management tools on the
target. Those tools are available in the :yocto_git:`meta-virtualization
</meta-virtualization>` layer.
You should set the :term:`PREFERRED_PROVIDER` for the Linux kernel to
``linux-dummy`` in a :term:`configuration file`::
PREFERRED_PROVIDER_virtual/kernel = "linux-dummy"
Otherwise an error is triggered. If desired, the
:term:`IMAGE_CONTAINER_NO_DUMMY` variable can be set to "1" to avoid triggering
this error.
The ``linux-dummy`` recipe acts as a Linux kernel recipe but builds nothing. It
is relevant to use as the preferred Linux kernel provider in this case as a
container image does not need to include a Linux kernel. Selecting it as the
preferred provider for the kernel will also decrease build time.
Using this class only deploys an additional ``tar.bz2`` archive to
:term:`DEPLOY_DIR_IMAGE`. This archive can be used in a container file (a file
typically named ``Dockerfile`` or ``Containerfile``). For example, to be used with
:wikipedia:`Podman <Podman>` or :wikipedia:`Docker <Docker_(software)>`, the
`container file <https://docs.docker.com/reference/dockerfile/>`__ could contain
the following instructions:
.. code-block:: dockerfile
FROM scratch
ADD ./image-container-qemux86-64.rootfs.tar.bz2 /
ENTRYPOINT /bin/sh
This is suitable to build a container using our generated root filesystem image.
.. _ref-classes-image-live:
``image-live``