Add Trivy vulnerability scanning for the Docker image

[vharseko]

Port of OpenIdentityPlatform/OpenDJ#854 — Docker image vulnerability scanning with Trivy, carrying the fixes from that PR's review (limit-severities-for-sarif, cache: false, no TRIVY_INSECURE, scanners: vuln, guarded uploads, unfixed CVEs reported for the published image).

build.yml — build-docker scans the freshly built image (resolved from the local Docker daemon, so the runner's linux/amd64 manifest only) right after the multi-server docker test. Findings do not fail the build: the SARIF report goes to code scanning via codeql-action/upload-sarif, so PRs get a "Code scanning results / trivy-build" check like CodeQL and the full list lives in the Security tab. Only fixable CRITICAL/HIGH CVEs are reported (ignore-unfixed: true plus limit-severities-for-sarif: true — without the latter the severity filter is silently dropped for SARIF output), and only the vulnerability scanner runs. The action's built-in ~1GB DB cache is disabled: the repo's actions cache is at 9.98 GiB of the 10 GB quota, with six ~690 MiB m2-repository entries the build-maven matrix depends on, and cache-trivy-* entries would LRU-evict them. The job gets security-events: write; SARIF upload also works for fork PRs on public repositories.

docker-scan.yml (new) — weekly cron (30 5 * * 1, CodeQL runs at 27 3) + workflow_dispatch scan of the published openidentityplatform/openam:latest: new CVEs surface in the already-released image (mostly via the tomcat:11-jre25 base) without any change in this repository. Unlike the build-time scan, unfixed CVEs are reported too. Category trivy-image-latest, distinct from trivy-build. The scheduled run is skipped in forks; manual runs are always allowed.

Differences from the OpenDJ version, by OpenAM's facts: one image (single Dockerfile, no alpine variant) so no matrix and one category each; image-ref reuses the existing OPENAM_IMAGE env; the ghcr.io copy of the same image is not scanned separately.

aquasecurity/trivy-action is pinned to ed142fd = v0.36.0 (resolved through the annotated tag; latest release), and the Dependabot github-actions config from #1134 will keep it current. Future accepted findings can be suppressed via a .trivyignore file in the repository root or dismissed in the Security tab.

Note: #1134 adds permissions: contents: read to the same build-docker job; whichever lands second gets a one-line conflict there (keep both contents: read and security-events: write).

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