Fix Dockerfile: single-stage build so gunicorn binary is in PATH

Dockerfile

@@ -1,18 +1,14 @@
-# Use multi-stage build for smaller image
-FROM python:3.11-slim AS builder
-
-WORKDIR /app
-COPY requirements.txt .
-RUN pip install --no-cache-dir -r requirements.txt
-
 FROM python:3.11-slim
+
 WORKDIR /app
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
     build-essential \
     && rm -rf /var/lib/apt/lists/*
 
-COPY --from=builder /usr/local/lib/python3.11/site-packages /usr/local/lib/python3.11/site-packages
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
 COPY . .
 
 ENV OPENBLAS_NUM_THREADS=1