Updates the JDK to JDK8

- necessary to allow seamless interop testing, as discussed earlier this week.

tools/dockerfile/grpc_java_base/Dockerfile

@@ -1,13 +1,21 @@
 # Base Dockerfile for the gRPC Java dev image
 FROM grpc/base
 
-RUN apt-get update && apt-get -y install java7-jdk
+# Install JDK 8
+#
+# TODO(temiola): simplify this if/when a simpler process is available.
+#
+RUN echo oracle-java8-installer shared/accepted-oracle-license-v1-1 select true | /usr/bin/debconf-set-selections
+RUN echo "deb http://ppa.launchpad.net/webupd8team/java/ubuntu trusty main" | tee /etc/apt/sources.list.d/webupd8team-java.list
+RUN echo "deb-src http://ppa.launchpad.net/webupd8team/java/ubuntu trusty main" | tee -a /etc/apt/sources.list.d/webupd8team-java.list
+RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys EEA14886
+RUN apt-get update && apt-get -y install oracle-java8-installer
 
 # Install maven
 RUN wget http://mirror.olnevhost.net/pub/apache/maven/binaries/apache-maven-3.2.1-bin.tar.gz && \
   tar xvf apache-maven-3.2.1-bin.tar.gz -C /var/local
 
-ENV JAVA_HOME /usr/lib/jvm/java-7-openjdk-amd64
+ENV JAVA_HOME /usr/lib/jvm/java-8-oracle
 ENV M2_HOME /var/local/apache-maven-3.2.1
 ENV PATH $PATH:$JAVA_HOME/bin:$M2_HOME/bin
 ENV LD_LIBRARY_PATH /usr/local/lib