Fix arm64 topn debsigner build context

[ibrahim halatci (ihalatci)]

Summary

Target branch: debian-topn.

The arm64 workflow clones develop into tooling, but builds the debsigner with tooling as its Docker context. The signer's COPY scripts /scripts therefore copies the unrelated top-level scripts and omits its /scripts/import_and_sign entrypoint.

Build from tooling/dockerfiles/debsigner instead. Docker selects the same default Dockerfile and copies the intended signer scripts. This is exactly one workflow line (+1/-1); amd64 behavior is unchanged.

Validation and scope

• Confirmed the signer Dockerfile and script layout on develop; the diff is exactly the approved context replacement and passes git diff --check with CRLF preserved.
• Runtime validation used only CI automatically triggered by the feature-branch push; no local Docker, manual dispatch, or rerun.
• Bullseye repository metadata failures are out of scope and will be handled separately. No platform removal or apt-validity bypass.
• No Dockerfile, tools-pin, bootstrap-guard backport, secret, or publication-gate changes. No stable publication, tags, or overwrite of previously published topn packages.

Automatic head CI results

Run 34240897064, head 6d906ead66c5164d7bcc5e7d2fd7c65c08ce4214: completed with failure, seven successful jobs and three failed jobs.

Jobs	Result	Evidence
All five amd64 platforms	Success	Existing amd64 path passed.
Bookworm arm64	Success	Signer image and package job passed; 12 topn/dbgsym arm64 packages for PostgreSQL 14-19 went through debsigs; log ends DEB signing finished successfully.
Jammy arm64	Success	Signer image and package job passed; 12 topn/dbgsym arm64 packages for PostgreSQL 14-19 went through debsigs; log ends DEB signing finished successfully.
Trixie arm64	Failure	Builder and signer images passed. Package output validation rejected dpkg-shlibdeps: warning: diversions involved - output may be incorrect (exit 1), before signing.
Noble arm64	Failure	Builder and signer images passed. Same package output-validation failure as Trixie (exit 1), before signing.
Bullseye arm64	Failure	Builder apt-get update rejected expired bullseye-security/InRelease metadata; signer and package steps skipped. Explicitly out of scope.

Bookworm and Jammy arm64 logs explicitly confirm Package publishing skipped since current branch is not equal to debian-topn. Other failures remain unchanged and are not addressed by this one-line signer-context PR.

References

• Related: Please provide an arm64 Docker image as well citus#8612
• Same nightly failure: https://github.com/citusdata/packaging/actions/runs/34216854535/job/102030690421
• Bootstrap guard already merged into develop: deb: skip pg_auto_failover community repo bootstrap on non-amd64 (unblock arm64 debs) #1216