docker/build-rootfs-image.sh: fix: size rootfs image from apparent size

[papamoose]

…ze, not allocated blocks (fixes mkfs.ext4 on ZFS data roots)

build-rootfs-image.sh sized the ext4 image from plain du, which on a compressed data root (e.g. ZFS) reports compressed sizes, the image ends up ~30% too small and mkfs.ext4 -d fails. Switching to du --apparent-size fixes it.

Before:

> [codeapi-sandbox sandbox-rootfs-image 7/7] RUN ... build-rootfs-image.sh /sandbox-rootfs /sandbox-rootfs.img:
29.87 __populate_fs: Could not allocate block in ext2 filesystem while writing file "magic.mgc"
29.87 mkfs.ext4: Could not allocate block in ext2 filesystem while populating file system

After:

#70 [sandbox-rootfs-image 7/7] RUN ... build-rootfs-image.sh /sandbox-rootfs /sandbox-rootfs.img
#70 33.42 e2fsck 1.47.0 (5-Feb-2023)
#70 34.05 resize2fs 1.47.0 (5-Feb-2023)
#70 34.18 Built ext4 rootfs image at /sandbox-rootfs.img (1.5G)
#70 DONE 34.4s