Update base debian:10.1-slim -> bullseye-20200224-slim

Includes security updates for:

openssl
libssl1.1
libidn2.0

Author: John McCabe

Dockerfile

@@ -3,7 +3,7 @@ ADD        . /go/src/github.com/proofpoint/certificate-init-container
 RUN        go install github.com/proofpoint/certificate-init-container && \
            go test github.com/proofpoint/certificate-init-container/...
 
-FROM debian:10.1-slim
+FROM debian:bullseye-20200224-slim
 
 COPY --from=0 /go/bin/certificate-init-container .
 USER 1000