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

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>

2 files changed, 65 insertions, 0 deletions

diff --git a/documentation/ref-manual/classes.rst b/documentation/ref-manual/classes.rst
index bc7ccb8b71..a6cf586dd5 100644
--- a/documentation/ref-manual/classes.rst
+++ b/documentation/ref-manual/classes.rst
@@ -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``
diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index 5b4561fc12..a67090308b 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -3837,6 +3837,24 @@ system and gives an overview of their function and contents.
       variable, see the :ref:`ref-classes-image_types`
       class file, which is ``meta/classes-recipe/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.