add BACKUP_TIMESTAMP to btcpay-backup.sh

[coinforensics]

I migrated the BACKUP_TIMESTAMP functionality to save the backup with datetime as part of the file name from the old backup.sh.

If my change is accepted, I plan to update the documentation to include information on the newly added environment variable.

[dennisreimann]

I remember we discussed adding those options, but went for the simplest approach which can then be extended with custom wrapper scripts. Free to open it up for dicussion again though.

[coinforensics]

I understand and like the importance of maintaining simplicity and consistency in the repository. The BACKUP_TIMESTAMP could provide a more convenient and automated approach to backup management, without the need for custom wrapper scripts (I personally don't like custom scripts in Docker-related processes).

[dennisreimann]

As said, I'm not opposing oppening it up for discussing — especially since I also use that functionality. Meanwhile, here's my wrapper script:

#!/bin/bash

BACKUP_FILE=/var/lib/docker/volumes/backup_datadir/_data/backup.tar.gz
BACKUP_DIR=/root/backups

cd /root/btcpayserver-docker
./btcpay-backup.sh

file=btcpay-$(date "+%Y%m%d-%H%M%S").tar.gz
cp $BACKUP_FILE $BACKUP_DIR/$file
rm $BACKUP_FILE
find $BACKUP_DIR/btcpay-* -mtime +14 -delete