tests: modernize dockerfile, upgrade to Ubuntu 19.04

test/docker/Dockerfile

@@ -1,31 +1,38 @@
 # keep this in sync with travis: https://docs.travis-ci.com/user/ci-environment
-FROM ubuntu:xenial
+FROM ubuntu:disco
 
 WORKDIR /root
 
 ENV DEBIAN_FRONTEND noninteractive
 
 # install basic deps
-RUN apt-get -y update
-RUN apt-get -y upgrade
-RUN apt-get -y install software-properties-common wget curl git xvfb sudo unzip python libxss1 net-tools libgtk-3-0
+RUN apt-get update && apt-get -y install --no-install-recommends \
+  software-properties-common wget curl git xvfb sudo unzip python libxss1 net-tools libgtk-3-0
 
 # install Java 11
-RUN add-apt-repository ppa:openjdk-r/ppa
-RUN apt-get update
-RUN apt install -y openjdk-11-jdk ca-certificates-java
-RUN java -version
+RUN add-apt-repository ppa:openjdk-r/ppa && \
+    apt-get update && \
+    apt install -y --no-install-recommends \
+    openjdk-11-jdk ca-certificates-java && \
+    java -version
 
 # dependencies needed by chromedriver
 RUN apt-get -y install libnss3 libgconf-2-4 libasound2
 
 # install latest lein
 ENV LEIN_ROOT 1
-RUN cd /usr/bin \
-    && wget https://raw.githubusercontent.com/technomancy/leiningen/stable/bin/lein \
-    && chmod a+x lein \
-    && lein
-RUN yes y | lein upgrade
+RUN cd /usr/bin && \
+    wget https://raw.githubusercontent.com/technomancy/leiningen/stable/bin/lein && \
+    chmod a+x lein && \
+    yes y | lein upgrade
 
 VOLUME ["/root/binaryage/dirac"]
 ENTRYPOINT ["/root/binaryage/dirac/scripts/docker-entrypoint.sh"]
+
+RUN echo "\n==============\nBuild summary:" && \
+    lsb_release -a && \
+    echo && \
+    java -version && \
+    lein --version && \
+    python --version && \
+    echo "==============\n"