Upgrade Docker image to Ubuntu 16.04 LTS, add bazel and deps.

This is in support of Bazelifying the build (envoyproxy#415).

ci/build_container/Dockerfile

@@ -1,3 +1,3 @@
-FROM ubuntu:trusty
+FROM ubuntu:xenial
 COPY ./build_container.sh /
 RUN ./build_container.sh

ci/build_container/build_container.sh

@@ -11,6 +11,13 @@ apt-get install -y gdb strace
 add-apt-repository -y ppa:ubuntu-toolchain-r/test
 apt-get update
 apt-get install -y g++-4.9
+# Bazel and related dependencies.
+apt-get install -y openjdk-8-jdk curl
+echo "deb [arch=amd64] http://storage.googleapis.com/bazel-apt stable jdk1.8" | tee /etc/apt/sources.list.d/bazel.list
+curl https://bazel.build/bazel-release.pub.gpg | apt-key add -
+apt-get update
+apt-get install -y bazel
+apt-get upgrade -y bazel
 rm -rf /var/lib/apt/lists/*
 
 # virtualenv