/usr/share/ubuntu-defaults-builder/template/hooks/chroot is in ubuntu-defaults-builder 0.49.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | #!/bin/sh
set -e
# Hook for building live images
#
# This script is run in the built chroot after all packages are installed,
# before it is packed into a squashfs. This is where you can apply extra tweaks
# to the live system.
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