docs(docker): document non-root runtime, data-volume migration, and image scan

docs/systems/security-scanning.md

@@ -14,6 +14,7 @@ later change once the remediation pass has cleared the backlog.
 | `.github/workflows/codeql.yml` | CodeQL SAST (`javascript-typescript`, build-mode none) | PR + push main + weekly | Security tab |
 | `.github/workflows/security.yml` | gitleaks (secrets, full history), OSV-Scanner (deps), Trivy config (IaC), Trivy license | PR + push main + weekly | Security tab |
 | `.github/workflows/scorecard.yml` | OpenSSF Scorecard (repo posture) | push main + weekly + on branch-protection change | Security tab + public badge |
+| `.github/workflows/docker-publish.yml` | Image scan (Trivy) + SBOM + provenance for the published container | tag push / manual | image scan (report-only) + SBOM + provenance |
 
 > **gitleaks findings** surface in the workflow's job log and PR summary — the
 > `gitleaks` job does not upload SARIF, so secret hits do **not** appear under
@@ -36,8 +37,10 @@ merge-blocking, Dependabot alerts, and native secret-scanning are GitHub *settin
   tag-move attacks and satisfies Scorecard's Pinned-Dependencies check.
 - `step-security/harden-runner` (egress-policy `audit`) on Linux jobs.
 - Least-privilege `permissions:` per workflow/job.
-- SBOM + SLSA provenance **will be** attached to the published container image
-  (added with the container-image-scan work in a later plan — not yet live).
+- SBOM + SLSA provenance are attached to the published container image at push
+  (`.github/workflows/docker-publish.yml`), alongside a report-only Trivy scan
+  of the amd64 image (the arm64 image ships unscanned; enforcement is turned
+  on in a later plan).
 
 ## Maintainer checklist (one-time GitHub settings — NOT code)