upgrade alpine base image from v3.20.3 to v3.21.0 including upgrade of postfix & ca-certificates package
https://www.postfix.org/announcements/postfix-3.9.1.html
https://git.alpinelinux.org/aports/commit/main/postfix?h=3.20-stable&id=068b58b02cb5a2c89cfe47352493249b51d1b58d
https://git.alpinelinux.org/aports/commit/main/ca-certificates?h=3.21-stable&id=537f890454d2213fd05cde5e458dfa9c8ef1977f