Upgrade to bionic, drop python 3.5 and add yapf

Dockerfile

@@ -1,5 +1,5 @@
 # vim:set ft=dockerfile:
-FROM ubuntu:xenial
+FROM ubuntu:bionic
 
 # explicitly set user/group IDs
 RUN groupadd -r postgres --gid=999 && useradd -r -d /var/lib/postgresql -g postgres --uid=999 postgres
@@ -9,26 +9,27 @@ RUN apt-get update && apt-get install -y software-properties-common locales && \
     rm -rf /var/lib/apt/lists/* && \
     localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8
 ENV LANG en_US.utf8
+ENV DEBIAN_FRONTEND noninteractive
 
 RUN apt-key adv --keyserver ha.pool.sks-keyservers.net --recv-keys B97B0AFCAA1A47F044F244A07FCC7D46ACCC4CF8
 
 ENV PG_MAJOR 10
 
-RUN echo 'deb http://apt.postgresql.org/pub/repos/apt/ xenial-pgdg main' $PG_MAJOR > /etc/apt/sources.list.d/pgdg.list
-
-RUN add-apt-repository ppa:jonathonf/python-3.6
+RUN echo 'deb http://apt.postgresql.org/pub/repos/apt/ bionic-pgdg main' $PG_MAJOR > /etc/apt/sources.list.d/pgdg.list
 
 RUN apt-get update \
-    && apt-get install -y postgresql-common \
+    && apt-get \
+        -o Dpkg::Options::="--force-confdef" \
+        -o Dpkg::Options::="--force-confold" \
+        install -y postgresql-common \
         sudo \
         libpq-dev \
-        python-pip \
-        python3.5-dev \
+        python3-pip \
         python2.7-dev \
         python3.6-dev \
         postgresql-$PG_MAJOR \
         postgresql-contrib-$PG_MAJOR \
-    && pip install tox
+    && pip3 install tox
 
 COPY ./ /dist