chore: lint and update docker-compose.yml

- Fixed port variable interpolation to use ${PORT:-8000} for a default value.
- Updated volume configuration to use a named volume (guestbook_data) mounted at /data.
- Improved YAML formatting for clarity.

production.Dockerfile

@@ -0,0 +1,14 @@
+FROM python:3.9-slim
+
+WORKDIR /app
+
+# Copy and install Python dependencies.
+COPY requirements.txt requirements.txt
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy the rest of the app code.
+COPY . .
+
+EXPOSE 5000
+
+CMD ["python", "app.py"]