Bump debian from 12.7-slim to 12.8-slim in /postfix #111

Job	Run time
Build and push PHP 7.4	4m 7s
Build and push nginx	30s
Build and push PHP 8.0	4m 31s
Build and push PHP 8.1	4m 35s
Build and push PHP 8.2	4m 47s
Build and push nginx-proxy	40s
Build and push PHP 8.3	4m 52s
Build and push postfix	1m 42s
Build and push PHP stable	4m 35s
Build and push redis	24s
Build and push cron	14s
Build and push mailhog	20s
Build and push mariadb	26s
Build and push newrelic-daemon	22s
	32m 5s

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