Update Dockerfile

Dockerfile

@@ -1,5 +1,5 @@
 # Use an Ubuntu base image
-FROM ubuntu:18.04
+FROM ubuntu:16.04
 
 # Set environment variables
 ENV NB_USER=jovyan
@@ -7,7 +7,7 @@ ENV NB_UID=1000
 ENV HOME=/home/${NB_USER}
 ENV JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64/
 
-# Install required system packages and Python 3.4
+# Install Python 3.4 and dependencies
 RUN apt-get update && apt-get install -y \
     software-properties-common \
     wget \