postinst is in postgresql-10-repack 1.4.2-2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 | #!/bin/sh
set -e
update-alternatives --install /usr/bin/pg_repack pg_repack /usr/lib/postgresql/10/bin/pg_repack 10
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