supermin: add ostree prepare-root config file

Bootc is looking for the prepare-root config file in the buildroot
environnement because the main assumption is that it's run from the
target container.
However, in osbuild, it's run from te buildroot, because podman inside
bwrap (inside supermin in our case) causes issues.
It's fine for RHCOS and SCOS where we use the target container as the
buildroot but we cannot do that for FCOS because we require python in
the buildroot.

For now, insert a prepare-root file in the supermin VM (use as the
buildroot for osbuild) until either :
- bootc learn to look into the container for it [1]
- we ship python in our images and can use them as buildroot.

Another approach would be to layer python and the osbuild dependencies
on top of our image and use that as the buildroot, but that would create
room for packages drift (what was in the repos at build time?). At least
using COSA it's easier to keep track of versions.

[1] bootc-dev/bootc#1410

Author: jbtrystram

src/osbuild-manifests/coreos.osbuild.x86_64.mpp.yaml

@@ -37,11 +37,11 @@ mpp-vars:
   # matches. Unfortunately for FCOS there is no python so we can't
   # really use FCOS as the buildroot so we'll use the host as the
   # buildroot there.
-  buildroot: "name:deployed-tree"
-    # mpp-if: osname in ['rhcos', 'scos', 'fc']
-    # then: "name:deployed-tree"
-    # else:
-    #   mpp-format-string: '{host_as_buildroot}'
+  buildroot:
+    mpp-if: osname in ['rhcos', 'scos']
+    then: "name:deployed-tree"
+    else:
+      mpp-format-string: '{host_as_buildroot}'
 mpp-define-images:
   - id: image
     sector_size:

src/supermin-init-prelude.sh

@@ -73,3 +73,13 @@ touch /etc/cosa-supermin
 # the missing link.  Hehe.
 update-alternatives --install /etc/alternatives/iptables iptables /usr/sbin/iptables-legacy 1
 update-alternatives --install /etc/alternatives/ip6tables ip6tables /usr/sbin/ip6tables-legacy 1
+
+# To build the disk image using osbuild and bootc install to-filesystem we need to
+# have a prepare-root config in the build environnement for bootc to read.
+# This workaround can be removed when https://github.com/bootc-dev/bootc/issues/1410
+# is fixed or we have python in all streams which allows us to use the OCI image as the buildroot.
+# Note that RHCOS and SCOS use the OCI as buildroot so they should not be affected by this.
+cat > /usr/lib/ostree/prepare-root.conf <<EOF
+[composefs]
+enabled = true
+EOF