feat(06-04): multi-stage Dockerfile with appuser + docker-compose runtime hardening (D-07/D-08/D-09)

- backend/Dockerfile: two-stage build (builder/runtime); runtime stage creates
  appuser uid=1000, copies installed packages from discardable builder stage,
  runs as non-root USER appuser; no --reload baked in CMD
- docker-compose.yml: backend + celery-worker get read_only:true,
  tmpfs:/tmp:mode=1777, cap_drop:ALL, security_opt:no-new-privileges:true;
  celery-beat intentionally left unhardened (Pitfall 7 — writes celerybeat-schedule)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/Dockerfile

@@ -1,17 +1,34 @@
-FROM python:3.12-slim
+# Stage 1: builder — compiles Python packages that need gcc
+FROM python:3.12-slim AS builder
 
-WORKDIR /app
+WORKDIR /build
 
-# System deps for PyMuPDF + OCR
-RUN apt-get update && apt-get install -y \
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    gcc \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir --prefix=/install -r requirements.txt
+
+# Stage 2: runtime — lean image with only what the app needs at runtime
+FROM python:3.12-slim AS runtime
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
     tesseract-ocr \
     libgl1 \
     libglib2.0-0 \
     && rm -rf /var/lib/apt/lists/*
 
-COPY requirements.txt .
-RUN pip install --no-cache-dir -r requirements.txt
+COPY --from=builder /install /usr/local
 
-COPY . .
+RUN groupadd --gid 1000 appgroup \
+    && useradd --uid 1000 --gid appgroup --shell /bin/sh --no-create-home appuser
+
+WORKDIR /app
+COPY --chown=appuser:appgroup . .
+
+USER appuser
 
 EXPOSE 8000
+
+CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]

docker-compose.yml

@@ -81,6 +81,13 @@ services:
         condition: service_healthy
       redis:
         condition: service_healthy
+    read_only: true
+    tmpfs:
+      - "/tmp:mode=1777"
+    cap_drop:
+      - ALL
+    security_opt:
+      - "no-new-privileges:true"
 
   celery-worker:
     build: ./backend
@@ -107,7 +114,15 @@ services:
         condition: service_healthy
       redis:
         condition: service_healthy
+    read_only: true
+    tmpfs:
+      - "/tmp:mode=1777"
+    cap_drop:
+      - ALL
+    security_opt:
+      - "no-new-privileges:true"
 
+  # celery-beat: NOT hardened — writes celerybeat-schedule to working directory (Phase 6 Pitfall 7).
   celery-beat:
     build: ./backend
     environment: