chore(docker): pin base images by digest (DEP-02)

[intendednull]

Why

Mutable tags bad. rust:latest / rust:slim / nginxinc/nginx-unprivileged:alpine change under foot. Build not reproducible. Tag takeover or malicious re-push = silent supply-chain swap. SEC DEP-02 medium.

Pin to digest. Build same bytes every time. Re-push of tag cannot poison.

What

Four Dockerfiles. Each FROM now tag@sha256:<digest>. Comment above each line records version + pin date + bump command.

File	Stage	Old	New
docker/relay.Dockerfile:2	builder	rust:latest	rust:1.95-slim-bookworm@sha256:caaf9ca7acd474892186860307d6f28e51fdbc1a4eada459fcff81517cf46a36
docker/relay.Dockerfile:8	runtime	rust:slim	same pin
docker/replay.Dockerfile:2	builder	rust:latest	same pin
docker/replay.Dockerfile:8	runtime	rust:slim	same pin
docker/storage.Dockerfile:2	builder	rust:latest	same pin
docker/storage.Dockerfile:8	runtime	rust:slim	same pin
docker/web.Dockerfile:2	builder	rust:latest	same pin
docker/web.Dockerfile:10	runtime	nginxinc/nginx-unprivileged:alpine	nginxinc/nginx-unprivileged:1.27-alpine@sha256:65e3e85dbaed8ba248841d9d58a899b6197106c23cb0ff1a132b7bfe0547e4c0

Rust 1.95 = current stable (matches rust-toolchain.toml channel = stable, latest stable as of 2026-04). Same image used for builder + runtime as before — no layout change.

Verify

Digests fetched live from Docker Hub:

docker buildx imagetools inspect rust:1.95-slim-bookworm
  → Digest: sha256:caaf9ca7acd474892186860307d6f28e51fdbc1a4eada459fcff81517cf46a36
docker buildx imagetools inspect nginxinc/nginx-unprivileged:1.27-alpine
  → Digest: sha256:65e3e85dbaed8ba248841d9d58a899b6197106c23cb0ff1a132b7bfe0547e4c0

Both pinned refs re-inspected by digest = resolve = real.

cargo fmt --check clean. cargo check --workspace clean (Dockerfile-only change, Rust untouched). Daemonless sandbox = no full docker build, but registry resolution proves digests valid.

Bump in future

docker buildx imagetools inspect rust:1.<minor>-slim-bookworm
docker buildx imagetools inspect nginxinc/nginx-unprivileged:1.<minor>-alpine

Take top-level Digest: line. Replace @sha256:... and update comment date.

Out of scope

• SBOM stage — issue body says skip.
• Switching runtime base to debian:bookworm-slim — separate change, not asked.
• USER directives — already present.

Refs #313

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