M41: harden trust boundaries and delivery

backend/Dockerfile

@@ -1,19 +1,26 @@
-FROM python:3.12-slim
+FROM python:3.12-slim-bookworm@sha256:a116514e19457bcb7af7efe9c3dd0b9b71e85b317694e7882a1c52aa15a78134 AS runtime-base
 ENV PYTHONDONTWRITEBYTECODE=1 PYTHONUNBUFFERED=1
 WORKDIR /app
-COPY backend/requirements.lock ./
-RUN pip install --no-cache-dir -r requirements.lock
+COPY backend/requirements-prod.lock ./
+RUN pip install --no-cache-dir -r requirements-prod.lock
 COPY backend/pyproject.toml ./
 COPY backend/app ./app
 COPY backend/alembic ./alembic
 COPY backend/alembic.ini ./
-COPY backend/tests ./tests
 COPY seed ./seed
 COPY knowledge ./knowledge
 COPY backend/entrypoint.sh ./entrypoint.sh
 RUN pip install --no-cache-dir --no-deps -e . && chmod +x ./entrypoint.sh \
     && addgroup --system app && adduser --system --ingroup app --home /app app \
     && chown -R app:app /app
+
+FROM runtime-base AS test
+COPY backend/requirements.lock ./requirements.lock
+RUN pip install --no-cache-dir -r requirements.lock
+COPY backend/tests ./tests
+USER app
+
+FROM runtime-base AS runtime
 # Run migrations and the API as an unprivileged user; nothing here needs root.
 USER app
 EXPOSE 8000