Apply suggestions from code review

Co-authored-by: Simon Li <orpheus+devel@gmail.com>

Dockerfile

@@ -2,9 +2,9 @@ FROM jupyter/base-notebook
 
 USER root
 
-RUN apt-get -y update > /dev/null \
- && apt-get -y upgrade > /dev/null \
- && apt-get -y install \
+RUN apt-get -y -q update \
+ && apt-get -y -q upgrade \
+ && apt-get -y -q install \
         dbus-x11 \
         firefox \
         xfce4 \
@@ -13,7 +13,6 @@ RUN apt-get -y update > /dev/null \
         xfce4-settings \
         xorg \
         xubuntu-icon-theme \
-    > /dev/null \
     # chown $HOME to workaround that the xorg installation creates a
     # /home/jovyan/.cache directory owned by root
  && chown -R $NB_UID:$NB_GID $HOME \
@@ -22,9 +21,9 @@ RUN apt-get -y update > /dev/null \
 # Install TurboVNC (https://github.com/TurboVNC/turbovnc)
 ARG TURBOVNC_VERSION=2.2.6
 RUN wget -q "https://sourceforge.net/projects/turbovnc/files/${TURBOVNC_VERSION}/turbovnc_${TURBOVNC_VERSION}_amd64.deb/download" -O turbovnc.deb \
- && apt-get install -y ./turbovnc.deb > /dev/null \
+ && apt-get install -y -q ./turbovnc.deb \
     # remove light-locker to prevent screen lock
- && apt-get remove -y light-locker > /dev/null \
+ && apt-get remove -y -q light-locker \
  && rm ./turbovnc.deb \
  && ln -s /opt/TurboVNC/bin/* /usr/local/bin/