feat(backend): add Dockerfile for taskapp-api image

backend/Dockerfile

@@ -0,0 +1,18 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libpq-dev gcc \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY app.py .
+
+EXPOSE 5000
+
+# Run init_db() once (creates the tasks table) then start gunicorn.
+# Same pattern as the first assignment's Dockerfile.
+CMD ["sh", "-c", "python -c 'from app import init_db; init_db()' && gunicorn --bind 0.0.0.0:5000 --workers 2 app:app"]