ci: Add a Containerfile-based workflow

This is a small but notable step towards making the build
process more container native.  The rpm-ostree bits
are hidden much more.  We use a special trick
specific to podman/buildah to do `FROM oci-archive`
on an oci-archive that was built in a previous image
stage.

This lets us do things not possible in a Containerfile
that is basically about total control over the image
layers:

 - Output content-addressed reproducible "chunked" layers
 - Choose whether or not to use zstd for layers

See discussion in e.g.
coreos/rpm-ostree#4688

Signed-off-by: Colin Walters <[email protected]>

.github/workflows/build-image.yml

@@ -13,10 +13,6 @@ jobs:
   build-image:
     runs-on: ubuntu-latest
 
-    container:
-      image: quay.io/centos-bootc/bootc-image-builder:latest
-      options: --privileged
-
     # Yes, this is a one-element matrix, but we may add c10s in the future soon
     strategy:
       matrix:
@@ -26,10 +22,24 @@ jobs:
             version: stream9
 
     steps:
+      - name: Update podman
+        run: |
+          # from https://askubuntu.com/questions/1414446/whats-the-recommended-way-of-installing-podman-4-in-ubuntu-22-04
+          ubuntu_version='22.04'
+          key_url="https://download.opensuse.org/repositories/devel:/kubic:/libcontainers:/unstable/xUbuntu_${ubuntu_version}/Release.key"
+          sources_url="https://download.opensuse.org/repositories/devel:/kubic:/libcontainers:/unstable/xUbuntu_${ubuntu_version}"
+          echo "deb $sources_url/ /" | sudo tee /etc/apt/sources.list.d/devel-kubic-libcontainers-unstable.list
+          curl -fsSL $key_url | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/devel_kubic_libcontainers_unstable.gpg > /dev/null
+          sudo apt update
+          sudo apt install -y podman
+
       - name: Checkout repository
         uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
 
       - name: Build
         run: |
-          rpm-ostree compose image --format=ociarchive \
-          --initialize ${{ matrix.os }}-bootc.yaml dest.oci-archive
+          podman build --security-opt=label=disable --cap-add=all --device /dev/fuse \
+            -t localhost/${{ matrix.os }}-${{ matrix.version }}-bootc -f Containerfile.${{ matrix.os }}-${{ matrix.version }}
+
+      - name: Run image
+        run: podman run --rm -ti localhost/${{ matrix.os }}-${{ matrix.version }}-bootc bootc --help

Containerfile.centos-stream9

@@ -0,0 +1,41 @@
+# This container build uses some special features of podman that allow
+# a process executing as part of a container build to generate a new container
+# image "from scratch".
+#
+# This container build uses nested containerization, so you must build with e.g.
+# podman build --security-opt=label=disable --cap-add=all --device /dev/fuse <...>
+#
+# # Why are we doing this?
+#
+# Today this base image build process uses rpm-ostree.  There is a lot of things that
+# rpm-ostree does when generating a container image...but important parts include:
+#
+# - auto-updating labels in the container metadata
+# - Generating "chunked" content-addressed reproducible image layers (notice
+#   how there are ~60 layers in the generated image)
+#
+# The latter bit in particular is currently impossible to do from Containerfile.
+# A future goal is adding some support for this in a way that can be honored by
+# buildah (xref https://github.com/containers/podman/discussions/12605)
+#
+# # Why does this build process require additional privileges?
+#
+# Because it's generating a base image and uses containerization features itself.
+# In the future some of this can be lifted.
+
+FROM quay.io/centos/centos:stream9 as repos
+
+FROM quay.io/centos-bootc/bootc-image-builder:latest as builder
+ARG MANIFEST=centos-bootc.yaml
+# XXX: we should just make sure our in-tree c9s repo points to the c9s paths and doesn't require vars to avoid these steps entirely
+COPY --from=repos /etc/dnf/vars /etc/dnf/vars
+COPY --from=repos /etc/yum.repos.d/centos.repo c9s.repo
+COPY --from=repos /etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial /etc/pki/rpm-gpg
+# rpm-ostree doesn't honor /etc/dnf/vars right now
+RUN for n in $(ls /etc/dnf/vars); do v=$(cat /etc/dnf/vars/$n); sed -ie s,\$${n},$v, c9s.repo; done
+RUN --mount=type=cache,target=/workdir --mount=type=bind,rw=true,src=.,dst=/buildcontext,bind-propagation=shared rpm-ostree compose image --cachedir=/workdir --format=ociarchive --initialize /buildcontext/${MANIFEST} /buildcontext/out.ociarchive
+
+FROM oci-archive:./out.ociarchive
+# Need to reference builder here to force ordering. But since we have to run
+# something anyway, we might as well cleanup after ourselves.
+RUN --mount=type=bind,from=builder,src=.,target=/var/tmp --mount=type=bind,rw=true,src=.,dst=/buildcontext,bind-propagation=shared rm /buildcontext/out.ociarchive