For the folks who might get the same issue, I find a way it works, though I don't know why. I change the work directory from /app to /workspace and it magically worked. Below is the Dockerfile:
FROM python:3.12-slim
# Create and set working directory explicitly
RUN mkdir -p /workspace
WORKDIR /workspace
COPY requirements.txt .
RUN pip install --upgrade pip setuptools wheel \
&& pip install -r requirements.txt
COPY . .
# Add debugging to see where we are and what files exist
RUN pwd && \
ls -la && \
echo "Current working directory contains:"
CMD ["python", "main.py"]