Update base image to buster

This commit updates the base image from debian 9.8 to 10.0.
Clair flags a number of high severity CVEs, some of which
are not fixed on stretch, but are addressed in buster (stable)
and sid (unstable).

Dockerfile

@@ -3,12 +3,7 @@ ADD        . /go/src/github.com/proofpoint/kapprover
 RUN        go install github.com/proofpoint/kapprover/cmd/kapprover && \
            go test github.com/proofpoint/kapprover/...
 
-FROM debian:9.8-slim
-
-RUN apt-get update && \
-    DEBIAN_FRONTEND=noninteractive apt-get install --assume-yes \
-                     # These packages are security updates
-                     tzdata=2019a-0+deb9u1
+FROM debian:10.0-slim
 
 COPY --from=0 /go/bin/kapprover .
 ENTRYPOINT ["/kapprover"]