Fix arm64 nightly debsigner build context

[ibrahim halatci (ihalatci)]

Root cause

The nightly arm64 debsigner build used the repository root as its Docker context. dockerfiles/debsigner/Dockerfile contains COPY scripts /scripts and ENTRYPOINT ["/scripts/import_and_sign"], but the intended signing scripts live in dockerfiles/debsigner/scripts, not the root scripts directory. The image therefore built successfully but failed when signing started because /scripts/import_and_sign was missing.

This was observed in the combined develop run, including the Bookworm arm64 job. All five non-Bullseye arm64 jobs reached this failure; amd64 succeeded.

Change

Change only the Build arm64 debsigner image command to use dockerfiles/debsigner as its build context. Docker selects the same default Dockerfile and copies the correct signing scripts. Exactly one file, one line changed (+1/-1). The arm64 condition and amd64 behavior are unchanged.

Validation and scope

• Confirmed the Dockerfile and signing-script layout, preserved line endings, and ran git diff --check.
• Runtime validation uses the automatically triggered push CI for this PR head; no local Docker build or manual dispatch/rerun.
• Bullseye's expired Debian repository metadata failures are explicitly out of scope and will be handled separately. No platform removal, apt-validity workaround, PG-version changes, tool-pin changes, or gate/secret changes.

Related: citusdata/citus#8612.