build: adopt chainguard busybox image as base for mango container

Chainguard is awesome.

This adopts the chainguard busybox image as the base image when building
mango's container/for release image builds.

Dockerfile

@@ -1,7 +1,7 @@
 FROM alpine:latest as certs
 RUN apk update && apk add ca-certificates
 
-FROM busybox:latest
+FROM cgr.dev/chainguard/busybox:latest
 COPY --from=certs /etc/ssl/certs /etc/ssl/certs
 
 COPY mango /usr/bin/mango