Upgrade Ubuntu to latest LTS (Bionic 18.04)

Upgrade Ubuntu to latest LTS (Bionic 18.04), including:
- Change the IMAGE used for base-notebook;
- Keep Mesos install from Mesosphere's Xenial repository, since they
  don't yet have a Bionic repository;
- gnupg needs to be installed for apt-key to work;
- --force-yes option to apt-get has been removed;
- Package libav-tools is replaced by ffmpeg;
- Update Let's Encrypt example script to use 18.04.

base-notebook/Dockerfile

@@ -1,9 +1,9 @@
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
 
-# Ubuntu 16.04 (xenial) from 2018-02-28
-# https://github.com/docker-library/official-images/commit/8728671fdca3dfc029be4ab838ab5315aa125181
-FROM ubuntu:xenial-20180228@sha256:e348fbbea0e0a0e73ab0370de151e7800684445c509d46195aef73e090a49bd6
+# Ubuntu 18.04 (bionic) from 2018-04-26
+# https://github.com/docker-library/official-images/commit/aac6a45b9eb2bffb8102353c350d341a410fb169
+FROM ubuntu:bionic-20180426@sha256:c8c275751219dadad8fa56b3ac41ca6cb22219ff117ca98fe82b42f24e1ba64e
 
 LABEL maintainer="Jupyter Project <jupyter@googlegroups.com>"
 

examples/docker-compose/bin/letsencrypt.sh

@@ -43,6 +43,6 @@ docker run --rm -it \
 # directory so that the FQDN doesn't have to be known later.
 docker run --rm -it \
   -v $SECRETS_VOLUME:/etc/letsencrypt \
-  ubuntu:16.04 \
+  ubuntu:18.04 \
 		bash -c "ln -s /etc/letsencrypt/live/$FQDN/* /etc/letsencrypt/ && \
 			find /etc/letsencrypt -type d -exec chmod 755 {} +"

pyspark-notebook/Dockerfile

@@ -23,13 +23,15 @@ RUN cd /tmp && \
 RUN cd /usr/local && ln -s spark-${APACHE_SPARK_VERSION}-bin-hadoop${HADOOP_VERSION} spark
 
 # Mesos dependencies
-RUN . /etc/os-release && \
+# Install from the Xenial Mesosphere repository since there does not (yet)
+# exist a Bionic repository and the dependencies seem to be compatible for now.
+RUN apt-get -y update && \
+    apt-get install --no-install-recommends -y gnupg && \
     apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv E56151BF && \
-    DISTRO=$ID && \
-    CODENAME=$VERSION_CODENAME && \
-    echo "deb http://repos.mesosphere.io/${DISTRO} ${CODENAME} main" > /etc/apt/sources.list.d/mesosphere.list && \
+    echo "deb http://repos.mesosphere.io/ubuntu xenial main" > /etc/apt/sources.list.d/mesosphere.list && \
     apt-get -y update && \
-    apt-get --no-install-recommends -y --force-yes install mesos=1.2\* && \
+    apt-get --no-install-recommends -y install mesos=1.2\* && \
+    apt-get purge --auto-remove -y gnupg && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/*
 

scipy-notebook/Dockerfile

@@ -6,9 +6,9 @@ LABEL maintainer="Jupyter Project <jupyter@googlegroups.com>"
 
 USER root
 
-# libav-tools for matplotlib anim
+# ffmpeg for matplotlib anim
 RUN apt-get update && \
-    apt-get install -y --no-install-recommends libav-tools && \
+    apt-get install -y --no-install-recommends ffmpeg && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/*