[jupyterhub] change workdir to /

jupyterhub/base/Dockerfile

@@ -83,5 +83,5 @@ RUN mkdir -p /usr/local/venv && \
 RUN chmod -x /etc/update-motd.d/*
 RUN mkdir /data
 
-WORKDIR "/root"
+WORKDIR "/"
 CMD [ "jupyterhub-singleuser" ]