Switched to msmtp

ssmtp is not available in buster, so we wouldn't be able to send mails.
Switched to msmtp, which serves the same purpose.

Dockerfile

@@ -5,7 +5,7 @@ RUN set -ex; \
 	apt-get update; \
 	\
 	apt-get install -y --no-install-recommends \
-		ssmtp \
+		msmtp \
 	; \
 	\
 	rm -rf /var/lib/apt/lists/* ;\