Update Dockerfile to use venv

Dockerfile

@@ -1,5 +1,9 @@
 FROM python:3.9
 
+# Set environment variables
+ENV VIRTUAL_ENV=/opt/venv
+ENV PATH="$VIRTUAL_ENV/bin:$PATH"
+
 WORKDIR /code
 
 COPY ./requirements.txt /code/requirements.txt
@@ -14,6 +18,9 @@ RUN pip install --upgrade pip \
 
 COPY . .
 
+# Set permissions
+RUN mkdir cache && chmod 777 cache
+
 EXPOSE 7860
 
 CMD ["shiny", "run", "app.py", "--host", "0.0.0.0", "--port", "7860"]