Default root config to mount sysroot as read only

See: https://github.com/fedora-iot/iot-distro/issues/81
     https://gitlab.com/fedora/bootc/base-images/-/blob/main/tier-0/ostree.yaml

Signed-off-by: Paul Whalen <pwhalen@fedoraproject.org>

fedora-iot-base.yaml

@@ -2,6 +2,8 @@ include:
   - bootupd.yaml
   # Enable composefs
   - composefs.yaml
+  # Read only sysroot
+  - sysroot-ro.yaml
 ref: fedora/rawhide/${basearch}/iot
 repos: []
 selinux: true

sysroot-ro.yaml

@@ -0,0 +1,15 @@
+# Set up default root config to mount sysroot as read only
+# https://fedoraproject.org/wiki/Changes/Silverblue_Kinoite_readonly_sysroot
+# See: https://gitlab.com/fedora/bootc/base-images/-/blob/main/tier-0/ostree.yaml
+# See: https://github.com/fedora-iot/iot-distro/issues/81
+
+postprocess:
+  - |
+    #!/usr/bin/env bash
+    set -xeuo pipefail
+
+    install -dm 0755 -o 0 -g 0 /usr/lib/ostree
+    cat >> /usr/lib/ostree/prepare-root.conf << 'EOF'
+    [sysroot]
+    readonly = true
+    EOF