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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: cebe8ff0508e0fc2de8378a1cf93eb8054e12699)

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 cb99d0b1c5
commit a4aa1bc237
2 changed files with 65 additions and 0 deletions
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47
documentation/ref-manual/classes.rst
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@@ -1118,6 +1118,53 @@ 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``

18
documentation/ref-manual/variables.rst
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@@ -3314,6 +3314,24 @@ system and gives an overview of their function and contents.
variable, see the :ref:`image_types <ref-classes-image_types>`
class file, which is ``meta/classes/image_types.bbclass``.
:term:`IMAGE_CONTAINER_NO_DUMMY`
When an image recipe has the ``container`` image type in
:term:`IMAGE_FSTYPES`, it expects the :term:`PREFERRED_PROVIDER` for
the Linux kernel (``virtual/kernel``) to be set to ``linux-dummy`` from a
:term:`configuration file`. Otherwise, an error is triggered.
When set to "1", the :term:`IMAGE_CONTAINER_NO_DUMMY` variable allows the
:term:`PREFERRED_PROVIDER` variable to be set to another value, thus
skipping the check and not triggering the build error. Any other value
will keep the check.
This variable should be set from the image recipe using the ``container``
image type.
See the documentation of the :ref:`ref-classes-image-container` class for
more information on why setting the :term:`PREFERRED_PROVIDER` to
``linux-dummy`` is advised with this class.
:term:`IMAGE_DEVICE_TABLES`
Specifies one or more files that contain custom device tables that
are passed to the ``makedevs`` command as part of creating an image.