Reorganize the Dockerfiles, to share code.
Also add gcc 6 to the 16.04 image and Bazel to the 15.10 image.
diff --git a/extras/dockerfiles/ubuntu-14.04_install.sh b/extras/dockerfiles/ubuntu-14.04_install.sh
new file mode 100644
index 0000000..d9628ae
--- /dev/null
+++ b/extras/dockerfiles/ubuntu-14.04_install.sh
@@ -0,0 +1,3 @@
+#!/bin/bash
+
+set -e