Update nmap sample-function to use new approach

I used AlpineFunction as the example to follow as suggested by @alexellis

Signed-off-by: scott.eppler <scott.eppler@coda.global>

sample-functions/Nmap/Dockerfile

-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