Update smtp-cli to v3.9

https://github.com/mludvig/smtp-cli/tree/v3.9

data/Dockerfiles/watchdog/Dockerfile

@@ -23,7 +23,7 @@ RUN apk add --update \
 	perl-mime-lite \
 	perl-term-readkey \
   tini \
-	&& curl https://raw.githubusercontent.com/mludvig/smtp-cli/v3.8/smtp-cli -o /smtp-cli \
+	&& curl https://raw.githubusercontent.com/mludvig/smtp-cli/v3.9/smtp-cli -o /smtp-cli \
 	&& chmod +x smtp-cli
 
 COPY watchdog.sh /watchdog.sh