Harden Docker image and release supply chain

[DenWin]

Parent: #31. Builds on #2-#7 and the completed layer refactor in #1.

Existing related work

• Evaluate and pin the production Debian base image #2 stable/digest-pinned Debian base
• Make downloaded Docker build dependencies reproducible #3 verified/pinned Bun, VTracer, and MarkItDown
• Run the ConvertX container as a non-root user #4 non-root runtime
• Audit and reduce Docker runtime dependencies #5 package/size audit
• Add BuildKit caching and multi-platform Docker validation #6 cache and multi-platform validation
• Improve container health checks and subprocess lifecycle handling #7 health/process lifecycle

This issue covers the remaining integrated supply-chain and runtime-hardening controls.

Required work

• Produce an SBOM for each released platform/image index.
• Scan OS and application dependencies with a documented severity/exception policy.
• Sign image manifests and publish verifiable provenance/attestations.
• Link release tags, source commit, workflow run, image digest, SBOM, and signature.
• Validate amd64 and arm64 manifests contain equivalent intended application versions.
• Set OCI source/version/revision/licenses/documentation metadata.
• Define immutable release tags and mutable convenience-tag policy.
• Add image-structure checks proving build tools/download archives are absent.
• Verify runtime user, writable paths, capabilities, read-only-root feasibility, and tmpfs requirements.
• Define seccomp/AppArmor guidance and required Linux capabilities.
• Add container smoke tests for startup, health, signal handling, and representative conversions.
• Establish rebuild/update response for critical base/package vulnerabilities.
• Document licensing implications of bundled converter packages.

Acceptance criteria

• Every release has an SBOM, scan result, signature, and provenance.
• Consumers can verify an image back to its source commit.
• Critical findings block release unless a reviewed, expiring exception exists.
• Runtime filesystem/capability assumptions are tested and documented.
• Multi-architecture images pass equivalent smoke checks.
• Existing Evaluate and pin the production Debian base image #2-Improve container health checks and subprocess lifecycle handling #7 acceptance criteria remain linked rather than duplicated.