Make the dist_install test use Java 8

This is necessary because the Kotlin support requires Java 8 or
higher.

Making this test use Java 8 required switching it to the java_stretch
Docker image and updating that image to include some Python
dependencies.

kokoro/linux/dist_install/build.sh

@@ -7,8 +7,8 @@ set -ex  # exit immediately on error
 # Change to repo root
 cd $(dirname $0)/../../..
 
-# Run tests under release docker image.
-export DOCKERFILE_DIR=kokoro/linux/64-bit
+export DOCKERHUB_ORGANIZATION=protobuftesting
+export DOCKERFILE_DIR=kokoro/linux/dockerfile/test/java_stretch
 export DOCKER_RUN_SCRIPT=kokoro/linux/pull_request_in_docker.sh
 export OUTPUT_DIR=testoutput
 export TEST_SET="dist_install"

kokoro/linux/dockerfile/test/java_stretch/Dockerfile

@@ -29,3 +29,10 @@ RUN apt-get install -y \
   # -- For java_linkage_monitor \
   openjdk-8-jdk \
   && apt-get clean
+
+# Python dependencies
+RUN apt-get install -y \
+  python-setuptools \
+  python-pip \
+  virtualenv \
+  && apt-get clean

tests.sh

@@ -104,7 +104,7 @@ build_dist_install() {
 
   # Try to install Java
   pushd java
-  use_java jdk7
+  use_java jdk8
   $MVN install
   popd