fix: pin the Python patch version and run the app as a non-root user

Two follow-ups to the review of the base image change.

1. Pin the patch version: `python:3.14-slim` is a floating tag, so a rebuild
   could silently pick up a different base image. It is now
   `python:3.14.10-slim`, so a rebuild is reproducible.

2. Run the app as an unprivileged user. The app only reads its own source
   and talks to Mongo over the network — it writes no files — so it does not
   need to be root. A `beer` system user is created and the image ends with
   `USER beer`.

base.Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.14-slim AS beer_base_image
+FROM python:3.14.10-slim AS beer_base_image
 
 WORKDIR /beer_data
 
@@ -12,8 +12,12 @@ COPY requirements.txt .
 # The Python version matters:
 #   - pymongo 4.19 requires Python >= 3.11, so the base image could not stay
 #     on 3.10 once the requirements were pinned.
-#   - 3.14 is the current stable release, so the image is not carrying the
-#     CVEs fixed in 3.11, 3.12 and 3.13.
+#   - 3.14 is a stable release (first released 2025-10-07, in its bugfix
+#     phase until 2030-10), so the image is not carrying the CVEs fixed in
+#     3.11, 3.12 and 3.13.
+#   - The patch version is pinned rather than the floating `3.14-slim` tag so
+#     a rebuild is reproducible and does not silently pick up a base image
+#     change.
 RUN pip install --upgrade pip --no-cache-dir
 
 RUN pip --default-timeout=1000 install --no-cache-dir -r requirements.txt

flask.Dockerfile

@@ -8,6 +8,12 @@ ADD src /beer_data/src
 # (``import brewing``), so that directory is the working directory.
 WORKDIR /beer_data/src/flask
 
+# Run as an unprivileged user.  The app only needs to read its own source
+# and talk to Mongo over the network — it writes no files — so it does not
+# need to be root.
+RUN useradd --system --create-home --home-dir /home/beer --shell /usr/sbin/nologin beer \
+    && chown -R beer:beer /beer_data
+
 # The app is served by waitress, not the Flask development server.
 # FLASK_DEBUG stays OFF: the old entrypoint ran the Werkzeug debugger in
 # production, which exposes an interactive console on any unhandled error.
@@ -21,6 +27,8 @@ ENV PORT=5000
 # session-signing key.
 # ENV SECRET_KEY=...
 
+USER beer
+
 EXPOSE 5000
 
 ENTRYPOINT ["python", "-m", "waitress", "--listen=0.0.0.0:5000", "--call", "beer_data:create_app"]