lab(lab6): Dockerize QuickNotes — multi-stage + compose + hardening

[GrandAdmiralBee]

Goal

Lab 6 submission — multi-stage Dockerfile for QuickNotes (≤25 MB target, lands 14.6 MB), distroless-static-nonroot runtime, compose with named-volume persistence + healthcheck, and all 6 hardening defaults applied + Trivy before/after.

Changes

• app/Dockerfile — multi-stage: golang:1.24.13-alpine builder → gcr.io/distroless/static-debian12:nonroot runtime. CGO_ENABLED=0, -trimpath -ldflags='-s -w', deps-before-source layer order, /data precreated nonroot for self-sufficient docker run.
• app/cmd/healthcheck/main.go — tiny static Go binary (~20 LoC, no deps) that hits /health and exits 0/1. Solves the "distroless has no shell" healthcheck problem from inside the image.
• compose.yaml — service quicknotes with named volume quicknotes-data:/data, healthcheck via ["CMD", "/healthcheck"], env defaults, restart: unless-stopped, port 127.0.0.1:8080:8080 (loopback), plus all 6 hardening defaults: user 65532:65532, read_only: true + tmpfs /tmp, cap_drop: [ALL], security_opt: [no-new-privileges:true].
• submissions/lab6.md — Dockerfile+compose paste, image inspect, smoketest, persistence cycle (PRESENT-1 → PRESENT-2 → GONE), all 7 design questions answered, 5 per-default verification commands captured, Trivy before/after table showing the supply-chain story of pinning vs auto-bump.

Testing

• docker build — final image 14.6 MB (under 25 MB cap).
• docker run --name qn-smoketest -p 18080:8080 quicknotes:lab6 + curl :18080/health → {"notes":4,"status":"ok"}.
• docker compose up --build -d → healthy after first interval; POST/GET/down/up/down -v cycle verified.
• 6 hardening verifications: User=nonroot:nonroot, sh not found (both forms), cap_drop expanded list, ReadonlyRootfs=true (+ Alpine sidecar proof), SecurityOpt=[no-new-privileges].
• Trivy: 0 OS HIGH/CRITICAL throughout; Go stdlib went 32 → 26 H+C after bumping 1.24.5 → 1.24.13; remaining 13×2 are fix-only-in-1.25.x → Go 1.24 EOL story documented.

Checklist

• Title is a clear sentence (≤ 70 chars)
• Commits are signed
• app/Dockerfile, compose.yaml, submissions/lab6.md all present
• CI on this PR will run on the existing Go 1.23/1.24 matrix (no workflow changes)