Use buster-20200514-slim for proxy base image
Our stretch images contain some libraries/utilities with CVEs. While we
can't yet upgrade all containers (see we can upgrade the proxy
image (which is the most widely deployed).
-ARG RUNTIME_IMAGE=debian:stretch-20190812-slim
+ARG RUNTIME_IMAGE=debian:buster-20200514-slim
-FROM debian:stretch-20190812-slim as fetch
+FROM debian:buster-20200514-slim as fetch
RUN apt-get update && apt-get install -y ca-certificates curl
WORKDIR /build
COPY bin/fetch-proxy bin/fetch-proxy
