docs: Add doc about build rpm-ostree+autosd

Myself and Eric we spend a bunch of time to create rpm-ostree
RPM for later inject it into autosd image. To avoid others to
spend time in such effort, here a documentation.

Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Eric Curtin <ecurtin@redhat.com>

docs/building-rpm-ostree-and-autosd.md

@@ -0,0 +1,169 @@
+<!-- markdownlint-disable-file MD013 MD033 -->
+# Building rpm-ostree in autosd image from scratch
+
+The AutoSD is an upstream repository for Red Hat In-Vehicle OS, much like CentOS Stream is to RHEL. AutoSD is based on CentOS Stream with a few divergences. If your base system is not a CentOS, you can use a container to generate a compatible CentOS Stream 9 RPM as showed below, otherwise just skip the step and use your raw system.
+
+## Building rpm-ostree rpm on CentOS Stream9
+
+Create a **ContainerFile** with the content below.
+
+```bash
+FROM centos:stream9
+
+RUN dnf update -y
+RUN dnf install -y 'dnf-command(config-manager)' epel-release
+RUN dnf config-manager --set-enabled crb
+RUN dnf config-manager --add-repo https://buildlogs.centos.org/9-stream/automotive/aarch64/packages-main/
+RUN dnf config-manager --add-repo https://buildlogs.centos.org/9-stream/autosd/aarch64/packages-main/
+
+# Install rpm-build and create empty dir structure.
+RUN dnf install -y rpm-build
+RUN mkdir -p $HOME/rpmbuild/{BUILD,RPMS,SOURCES,SPECS,SRPMS}
+
+# Install deps required for recent rpm-ostree
+RUN cd $HOME/rpmbuild/SPECS/ && curl -OL https://gitlab.com/redhat/centos-stream/rpms/rpm-ostree/-/raw/c9s/rpm-ostree.spec
+
+RUN cd $HOME/rpmbuild/SPECS && dnf builddep -y rpm-ostree.spec
+
+# Lets use the latest from community (see next curl rustup step).
+# Also avoids "cargo metadata: error: failed to run rustc to learn about target-specific information"
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+```
+
+## Building the image
+
+```bash
+podman build -t rpmostreedevel -f ContainerFile
+```
+
+## Running the image
+
+The strategy now is run the container sharing the current **HOME/rpmbuild** dir, so as soon the rpm is created in the container it will be located in the host machine in the running user dir.
+
+```bash
+sudo podman run --rm --security-opt label=disable -v /home/$USER/rpmbuild/:/home/$USER/rpmbuild/ -ti rpmostreedevel /bin/bash
+
+[root@d1ec4ed543ec /]#
+```
+
+## Building a forked rpm-ostree
+
+Now time to clone your [rpm-ostree](https://github.com/coreos/rpm-ostree) fork and build it.
+
+Cloning the repo and switch to your work branch
+
+```bash
+cd /tmp
+git clone https://github.com/dougsland/rpm-ostree.git && cd rpm-ostree
+git checkout remotes/origin/initoverlayfs -b initoverlayfs
+```
+
+Adding required submodules for the build
+
+```bash
+git submodule update --init
+```
+
+Generating source code
+
+```bash
+cd packaging
+cargo install cargo-vendor-filterer --version ^0.5
+make -f Makefile.dist-packaging
+cp rpm-ostree-e2dbedd3.tar.xz $HOME/rpmbuild/SOURCES/
+
+# Getting version and updating the spec file
+rpmostree_tarxz=$(ls rpm-ostree-*.tar.xz| sed 's/rpm-ostree-\(.*\)\.tar\.xz/\1/')
+sed -i "/^Version:/s/:.*/: $rpmostree_tarxz/" $HOME/rpmbuild/SPECS/rpm-ostree.spec
+
+# Remove any specific Patch for Fedora koji build
+# See:
+# https://gitlab.com/redhat/centos-stream/rpms/rpm-ostree/-/blob/8e861dccda62f170461da2b90cc2f549886094aa/0001-cliwrap-rpm-mark-eval-E-as-safe.patch#L14
+# https://gitlab.com/redhat/centos-stream/rpms/rpm-ostree/-/blob/8e861dccda62f170461da2b90cc2f549886094aa/0001-cliwrap-rpm-mark-eval-E-as-safe.patch
+sed -i '/Patch[0-9]*:/d' $HOME/rpmbuild/SPECS/rpm-ostree.spec
+```
+
+Keep in mind, the version of rpm-ostree must be higher than
+the current version shipped in the autosd image. In the time this document was created the source file generated in the rpm-ostree is based on hash commit which is not enough most of the times. Below a quick hack.
+
+```bash
+pushd $HOME/rpmbuild/SOURCES/
+    # untar the source file:
+    tar xvf rpm-ostree-e2dbedd3.tar.xz
+
+    # rename the dir
+    mv rpm-ostree-e2dbedd3 rpm-ostree-2025.11
+
+    # tar again
+    tar -cJf rpm-ostree-2025.11.tar.xz rpm-ostree-2025.11
+popd
+```
+
+Change the spec file to use this new version, replace in the
+spec file the Version field to Version: **2025.11**
+
+```bash
+vi /root/rpmbuild/SPECS/rpm-ostree.spec
+```
+
+Finally generate the RPM
+
+```bash
+rpmbuild -ba ~/rpmbuild/SPECS/rpm-ostree.spec
+```
+
+You are done, exit from the container
+
+```bash
+exit
+```
+
+## Building autosd with the generated file
+
+Clone the autosd repo
+
+```bash
+git clone https://gitlab.com/CentOS/automotive/sample-images && cd sample-images
+```
+
+Create a RPM repo in rpmbuild/RPMS, now it includes the fresh rpm-ostree
+
+```bash
+pushd ~/rpmbuild/RPMS
+    createrepo .
+popd
+```
+
+Add the new RPM repo just created into the sample-images project
+
+```bash
+vi osbuild-manifests/distro/cs9.ipp.yml
+  - id: extra
+   baseurl: file:///home/curtine/rpmbuild/RPMS/
+```
+
+Full diff Example:
+
+```bash
+diff --git a/osbuild-manifests/distro/cs9.ipp.yml b/osbuild-manifests/distro/cs9.ipp.yml
+index c2e3abc..1564c9d 100644
+--- a/osbuild-manifests/distro/cs9.ipp.yml
++++ b/osbuild-manifests/distro/cs9.ipp.yml
+@@ -15,6 +15,8 @@ mpp-vars:
+     baseurl: https://mirror.stream.centos.org/SIGs/9-stream/autosd/$arch/packages-main/
+   - id: next
+     baseurl: https://download.copr.fedorainfracloud.org/results/@centos-automotive-sig/next/epel-9-$arch/
++  - id: extra
++    baseurl: file:///root/rpmbuild/RPMS/
+   distro_devel_repos:
+   - id: crb
+     baseurl: $distro_baseurl/CRB/$arch/os/
+```
+
+Generate the autosd image using initoverlayfs + rpm-ostree
+
+```bash
+pushd osbuild-manifests
+     sudo make cs9-qemu-initoverlayfs-ostree.x86_64.qcow2
+popd
+```