nextcloud: Run an SMTP relay locally

For some reason, Nextcloud seems to have trouble sending mail via the
network-wide relay.  It opens a connection, then just sits there and
never sends anything until it times out.  This happens probably 4 out of
5 times it attempts to send e-mail messages.

Running Postfix locally and directing Nextcloud to send mail through it
and then on to the network-wide relay seems to work much more reliably.

hosts

@@ -235,6 +235,9 @@ chromie.pyrocufflink.blue
 [smtp-relay]
 smtp1.pyrocufflink.blue
 
+[smtp-relay:children]
+nextcloud
+
 [squid]
 
 [sudo]