fix: add missing VOLUME directive

Dockerfile

@@ -21,6 +21,7 @@ RUN pip3 install --no-cache-dir -r $APP_CODE/requirements.txt
 RUN useradd -m -u 1000 user
 
 # Copy our code to $APP_CODE taking car of the permissions
+
 COPY --chown=user:user --chmod=u=rX src/ $APP_CODE/
 COPY --chown=user:user --chmod=u=rX .streamlit/ $APP_HOME/.streamlit/
 
@@ -29,6 +30,7 @@ COPY --chown=user:user --chmod=u=rX .streamlit/ $APP_HOME/.streamlit/
 # $ docker ... --mount type=volume,src=ai-playground-vol,dst=/data ...
 # 
 # see https://huggingface.co/docs/hub/spaces-storage
+VOLUME /data
 RUN mkdir -p /data && chown root:user /data && chmod ug+rwX /data
 
 # User