Rework build system further

- Move everything related to base/ into that directory, including Containerfile
- Introduce `bootc-base-image-rebuild-self` that can be used to
  resynthesize the base image, and it has docs.
- Rework the standard image to put its build instructions in
  usr/share/doc/bootc-image-standard to serve as a reference

Signed-off-by: Colin Walters <walters@verbum.org>

base/Containerfile

@@ -0,0 +1,46 @@
+# This is a relatively minimal base image build; it's intended as a derivation
+# point.
+#
+# This container build uses nested containerization to construct
+# a target rootfs from scratch; so you must build with e.g.
+# podman build --security-opt=label=disable --cap-add=all --device /dev/fuse <...>
+
+# If you want to configure the input rpm-md repositories, just override this
+# container image.
+FROM quay.io/fedora/fedora:rawhide as repos
+
+# We use stream10 to demonstrate that we support "cross builds".
+FROM quay.io/centos/centos:stream10 as builder
+RUN <<EORUN
+set -xeuo pipefail
+# For rpm-ostree v2025.5
+curl -L -o /etc/yum.repos.d/coreos-continuous.repo https://copr.fedorainfracloud.org/coprs/g/CoreOS/continuous/repo/centos-stream-10/group_CoreOS-continuous-centos-stream-10.repo
+dnf -y install rpm-ostree selinux-policy-targeted sqlite
+EORUN
+# Copy in our source code.
+COPY . /src
+WORKDIR /src
+RUN --mount=type=bind,from=repos,src=/,dst=/repos,rw <<EORUN
+set -xeuo pipefail
+# Copy the build configuration into the builder image, as if it's the final image
+cp -a . /usr/lib/sysimage/base-image-manifest
+# And embed the rebuild script
+install -m 0755 -t /usr/libexec ./bootc-base-image-rebuild-self
+# Finally, run the build script in the same way we expect custom images to do.
+/usr/libexec/bootc-base-image-rebuild-self /repos /target-rootfs
+EORUN
+
+# This pulls in the rootfs generated in the previous step
+FROM scratch
+COPY --from=builder /target-rootfs/ /
+LABEL containers.bootc 1
+# This is an ad-hoc way for us to reference bootc-image-builder in
+# a way that in theory client tooling can inspect and find. Today
+# it isn't widely used.
+LABEL bootc.diskimage-builder quay.io/centos-bootc/bootc-image-builder
+# https://pagure.io/fedora-kiwi-descriptions/pull-request/52
+ENV container=oci
+# Make systemd the default
+STOPSIGNAL SIGRTMIN+3
+CMD ["/sbin/init"]
+