Add an entrypoint to configure ssmtp

Dockerfile

@@ -1,6 +1,7 @@
 FROM alpine
 
 RUN apk add --no-cache \
+    ca-certificates \
     nagios \
     nagios-plugins \
     nagios-plugins-dig \
@@ -14,8 +15,12 @@ RUN apk add --no-cache \
     nagios-plugins-ssh \
     nagios-plugins-ssl_validity \
     nagios-web \
-    nrpe-plugin
+    nrpe-plugin \
+    ssmtp
 
 USER nagios
 
+COPY entrypoint.sh /docker-entrypoint.sh
+ENTRYPOINT ["/docker-entrypoint.sh"]
+
 CMD ["/usr/sbin/nagios", "/etc/nagios/nagios.cfg"]