[DEP-02] Docker images use unpinned `rust:latest` / `rust:slim` / `nginx:alpine` tags

[intendednull]

Audit finding from #300 (commit 679f9fe)

Severity: medium
Category: dependencies / supply chain
File: docker/relay.Dockerfile:1,6 ; replay.Dockerfile:1,6 ; storage.Dockerfile:1,6 ; web.Dockerfile:1,8
Obvious fix: yes (tag pinning); no (USER + SBOM require app-side validation)

Description

All four Dockerfiles use mutable tags (rust:latest, rust:slim, nginx:alpine) rather than digest-pinned images (@sha256:...). Builds are non-reproducible and are vulnerable to upstream image takeover or a malicious tag re-push. There is no SBOM stage.

Impact / Threat

A malicious or buggy upstream image push silently lands on the next build; production loses the ability to reconstruct the exact image bits a release was built from.

Suggested fix

Pin each FROM to a rust:1.<minor>-slim-bookworm@sha256:<digest> and nginx:1.27-alpine@sha256:<digest>; consider adding a docker buildx --sbom=true step or syft SBOM generation in CI.

Verify

grep -nE '^FROM ' docker/*.Dockerfile