qpdf expands argument files before parsing options, so a bare positional password
beginning with @ was resolved as a path and the file's contents became the
encryption key: exit 0, and the user's own password no longer opened the PDF.
Drive the encrypt through a job-JSON file so neither password reaches argv. The
=-joined flag form needs qpdf 11.7+, and the released image carries 11.3.0 which
rejects it; job JSON works on both (verified 11.3.0 and 12.1.0, R = 6 each).