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.
Interpolation can and does insert % placeholders into the printf
template text - although not valid ones as they're intended for
python.
So be more careful! Put all inserted text into the parameters to
printf, or use echo.
Also, keep some of the alterations used whilst diagnosing this.
