Fix Dockerfile: single-stage build

Dockerfile

@@ -1,12 +1,9 @@
-FROM python:3.12-slim AS builder
-WORKDIR /app
-COPY requirements.txt .
-RUN pip install --no-cache-dir -r requirements.txt
-
 FROM python:3.12-slim
 WORKDIR /app
 RUN addgroup --system app && adduser --system --group app
-COPY --from=builder /usr/local/lib/python3.12/site-packages /usr/local/lib/python3.12/site-packages
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt && \
+    rm -rf /root/.cache/pip
 COPY . .
 USER app
 EXPOSE 7860