update dockerfile: separate installing dependencies from copying the files to reduce build time

Dockerfile

@@ -1,5 +1,9 @@
 FROM python:3.12.7-slim
 WORKDIR /app
+
+COPY requirements.txt /app
+RUN pip install --no-cache-dir -r requirements.txt
+
 COPY . /app
-RUN pip install -r requirements.txt
+
 CMD ["sh", "-c", "python app.py & python src/evaluator/run_evaluator.py"]