Disk image: btrfs root from the config, not --rootfs

bootc-image-builder panics on --rootfs btrfs with a disk size.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Taskfile.yml

@@ -155,7 +155,7 @@ tasks:
       - task: export-to-storage
         vars: { IMAGES: "{{.IMAGE}}" }
       - mkdir -p {{.OUTPUT_DIR}}
-      # --rootfs btrfs: what Aurora installs with, as in build-installer-iso.
+      # Root on btrfs, as Aurora installs it: disk/config.toml.
       - >-
         {{.SUDO}} {{.CONTAINER_RUNTIME}} run --rm --privileged {{.PULL_FLAG}}
         --security-opt label=type:unconfined_t
@@ -164,7 +164,6 @@ tasks:
         -v {{.STORAGE}}:/var/lib/containers/storage
         {{.BIB_IMAGE}}
         --type qcow2
-        --rootfs btrfs
         localhost/{{.IMAGE}}
     generates:
       - "{{.OUTPUT_DIR}}/qcow2/disk.qcow2"

disk/config.toml

@@ -1,8 +1,19 @@
 # bootc-image-builder's config for the disk image (task build-disk-image).
 # No user: the image has none, so plasma-setup (Aurora's, enabled by
-# default) creates the first one at first boot. Only the disk's minimum
+# default) creates the first one at first boot.
-# size (customizations.filesystem isn't supported for disk images); a
+#
-# qcow2 is sparse, so the download is the size of what's written, not 64
+# A 64 GiB disk, root on btrfs as Aurora installs it. qcow2 is sparse, so
-# GiB.
+# the download is the size of what's written. The btrfs partition goes here
+# rather than in --rootfs: bootc-image-builder panics on --rootfs btrfs
+# with a disk size ("unknown fs type: btrfs for /"), and doesn't support
+# customizations.filesystem for disk images.
 [customizations.disk]
 minsize = "64 GiB"
+
+[[customizations.disk.partitions]]
+type = "btrfs"
+minsize = "60 GiB"
+
+[[customizations.disk.partitions.subvolumes]]
+name = "root"
+mountpoint = "/"