/usr/lib/rootstrap/modules/mkisofs is in rootstrap 0.3.25-1.
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 | #!/bin/sh -e
if test -z "$output"; then
output="rootstrap.iso"
fi
mkisofs -R -q -o $WORKDIR/$output $options $TARGET
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