chore: container hardening — read-only root FS + tmpfs /tmp + Python env

Dockerfile

@@ -53,7 +53,14 @@ COPY --chown=app:app src/ src/
 USER app
 
 # Put the venv on PATH so `uvicorn` resolves without `uv run` indirection.
-ENV PATH="/app/.venv/bin:${PATH}"
+# PYTHONDONTWRITEBYTECODE=1 stops Python from attempting `.pyc` writes under
+# `__pycache__/` on cold start — they would EROFS-fail under the read-only
+# root FS configured in docker-compose.yml. PYTHONUNBUFFERED=1 keeps
+# uvicorn's stdout from being held behind line-buffering when running under
+# non-TTY container stdio.
+ENV PATH="/app/.venv/bin:${PATH}" \
+    PYTHONDONTWRITEBYTECODE=1 \
+    PYTHONUNBUFFERED=1
 
 EXPOSE 8000
 

docker-compose.yml

@@ -23,6 +23,15 @@ services:
       - OTEL_EXPORTER_OTLP_ENDPOINT=http://jaeger:4317
       - OTEL_EXPORTER_OTLP_PROTOCOL=grpc
       - OTEL_SERVICE_NAME=harness-python-react
+    # Container hardening. The root FS is read-only at the kernel level so a
+    # post-exploit shell can't modify /app, persist binaries, or fill disk
+    # under the `app` user's ownership. /tmp is the only writable path —
+    # tmpfs-mounted with a 64 MB ceiling so it can't be abused as unbounded
+    # storage. Verified: `touch /app/foo` → EROFS; `touch /tmp/foo` succeeds;
+    # healthcheck reports healthy. See docs/SECURITY.md "Container Security".
+    read_only: true
+    tmpfs:
+      - /tmp:size=64m,mode=1777
 
   frontend:
     build: ./frontend

docs/SECURITY.md

@@ -61,8 +61,16 @@ Tag v*.*.*       ──►  release.yml: build image, push to ghcr.io, generate
 - **Builder** — runs `uv sync --frozen --no-dev`. Has uv, pip cache, build tools.
 - **Runtime** — `python:3.14-slim`, copies only `.venv` + `src/` from the builder, runs as non-root user `app`. No uv, no pip cache, no build tools, no dev deps.
 
+Runtime stage env: `PYTHONDONTWRITEBYTECODE=1` (no `.pyc` writes — would EROFS-fail under the read-only root FS) and `PYTHONUNBUFFERED=1` (uvicorn stdout flushed immediately).
+
+`docker-compose.yml`'s `app` service runs with `read_only: true` and a `tmpfs: /tmp:size=64m,mode=1777` mount. The kernel rejects writes to every path except the 64 MB tmpfs, so a post-exploit shell under the `app` user cannot modify `/app`, persist binaries, or fill the host's disk under `app`'s ownership. Verified locally: `touch /app/foo` → `Read-only file system`; `touch /tmp/foo` succeeds; healthcheck reports `healthy`.
+
 Healthcheck uses stdlib `urllib.request` so curl isn't in the image.
 
+### Distroless evaluation — deferred
+
+`gcr.io/distroless/python3-debian12` ships Python at `/usr/bin/python3` while the current builder stage materialises a venv whose `pyvenv.cfg` and interpreter symlinks reference `/usr/local/bin/python3.14` (Dockerfile comment makes this constraint explicit). Migrating requires either matching Python paths between stages (no distroless variant matches slim's `/usr/local`) or rebuilding the venv inside the runtime stage (distroless has no `pip` / `uv`). Either route adds engineering risk and operational friction (no `docker exec ... sh`) that outweighs the marginal attack-surface reduction now that read-only-FS + non-root + no-build-tools + trivy-scanning are all in place. Revisit when distroless ships a `/usr/local` variant or when the venv-in-runtime cost shrinks.
+
 ## What's intentionally out of scope (scaffold)
 
 - **WAF / DDoS** — deployment-environment concerns, not template concerns.