build: Upgrade base image for deployment dockerfile

Debian 8.x has unpatched security vulnerabilities in the glibc
package (even though it's supposed to be in its LTS period?), so
upgrade to the current 9.x.

https://security-tracker.debian.org/tracker/source-package/glibc

Release note (build change): Release Docker images are now built on
Debian 9.8.

build/deploy/Dockerfile

@@ -1,10 +1,13 @@
-FROM debian:8.11
+FROM debian:9.8-slim
 
-# Install root CAs so we can make SSL connections to phone home and
-# do backups to GCE/AWS/Azure.
+# For deployment, we need
+# libc6 - dynamically linked by cockroach binary
+# ca-certificates - to authenticate TLS connections for telemetry and
+#                   bulk-io with S3/GCS/Azure
+# tzdata - for time zone functions
 RUN apt-get update && \
 	apt-get -y upgrade && \
-	apt-get install -y ca-certificates  && \
+	apt-get install -y libc6 ca-certificates tzdata && \
 	rm -rf /var/lib/apt/lists/*
 
 RUN mkdir -p /cockroach