Fix Docker distro version mismatch for Bazel

This fixes an error that was manifesting as:

cp: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.25' not found (required by cp)

The error was being caused by a mismatch between the container that was
being pulled in container_pull and the debian dependencies collected.
This change fixes the error by specifying which distro version to pull
(as opposed to the previous default of "latest").

Change-Id: Ib1149643241227e3cf4ffd25be069124c327f59f
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
diff --git a/WORKSPACE b/WORKSPACE
index 9bfb716..743fcf3 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -68,6 +68,7 @@
     name = "debian_base",
     registry = "index.docker.io",
     repository = "library/debian",
+    tag = "stretch",
 )
 
 # rules for installing packages from debian snapshot