templates/docker-compose/ - update Borgmatics image

It's necessary to update to the `latest` image because the
`latest-msmtp` image is now unmaintained and doesn't support postgres
17.

However, this means we need to adjust things (the backup script and
the environment variable config) because we need to use the latest
notification mechanism, `apprise` instead of msmtp.

Tested, seems to be working.

templates/docker-compose/docker-compose.yml

@@ -260,20 +260,18 @@ services:
       - ./volumes/borg-ssh-config:/root/.ssh/ # ssh keys; sshd writes knownhosts etc here
 
     environment:
+      # Work around the use of a fancy init system s6:
+      # https://github.com/borgmatic-collective/docker-borgmatic/issues/320#issuecomment-2089003361
+      S6_KEEP_ENV: 1
+      
       POSTGRES_USER: nextcloud
       POSTGRES_DB: nextcloud
       POSTGRES_HOST: postgres
       BORG_ARCHIVE: nick
       BORG_ARCHIVE_LABEL: snackpot
-      MAIL_RELAY_HOST: mail.noodlefactory.co.uk
-      MAIL_PORT: 25
-      MAIL_AUTH_METHOD: login
-      MAIL_STARTTLS: 'on'
-      MAIL_USER: nc.noodlefactory.co.uk
-      MAIL_FROM: borgmatic@snackpot.noodlefactory.co.uk
-      MAIL_TO: nick@noodlefactory.co.uk
-      MAIL_SUBJECT: Borgmatic Backup
-      # MAIL_PASSWORD is set via volumes/borgmatic-config/msmtp.env, created via ansible
+      APPRISE_URI: "mailtos://mail.noodlefactory.co.uk:25?user=nc.noodlefactory.co.uk&from=borgmatic@snackpot.noodlefactory.co.uk&to=nick@noodlefactory.co.uk"
+      # SMTP_PASSWORD is set via borgmatic.env, created via ansible,
+      # and appended to APPRISE_URL by borgmatic/backup.sh script
       # Test SMTP auth on the server https://doc.dovecot.org/admin_manual/debugging/debugging_authentication/
     env_file:
       - ./borgmatic.env