Update nmap sample-function to use new approach
I used AlpineFunction as the example to follow as suggested by
-FROM functions/alpine:latest
+FROM openfaas/classic-watchdog:0.14.4 as watchdog
+
+FROM alpine:3.10
+
+COPY --from=watchdog /fwatchdog /usr/bin/fwatchdog
+RUN chmod +x /usr/bin/fwatchdog
RUN apk add --no-cache nmap
