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/bootc-base-image-rebuild-self

@@ -0,0 +1,17 @@
+#!/bin/bash
+# This script regenerates this base image using a build
+# configuration (list of packages, scripts) embedded in this current image.
+# The actual *content* packages will come from the source root.
+set -xeuo pipefail
+source_root=$1
+shift
+target=$1
+shift
+if ! test -x /usr/bin/rpm-ostree; then
+    dnf -y install rpm-ostree
+fi
+rpm-ostree experimental compose rootfs --source-root-rw=$source_root /usr/lib/sysimage/base-image-manifest/manifest.yaml $target
+# Finally, propagate the configuration and build script into the target root.
+for f in /usr/lib/sysimage/base-image-manifest /usr/libexec/bootc-base-image-rebuild-self; do
+    cp -a $f $target/$f
+done