pdftk not found in Drupal 11

Problem/Motivation

The module fillpdf uses the command pdftk in tests. Tests work fine on Drupal 10 and earlier. In the Drupal 11 environment, they fail with this error:

Symfony\Component\Process\Exception\ProcessStartFailedException: The command "'pdftk' '--version'" failed.
Working directory: /builds/project/fillpdf/web
Error: proc_open(): posix_spawn() failed: No such file or directory

Steps to reproduce

See phpunit test results in this merge request:

https://git.drupalcode.org/project/fillpdf/-/merge_requests/53/pipelines

Note that all tests pass except phpunit next major.

Proposed resolution

I speculate that pdftk needs to be installed in the Drupal 11 container.