upgrade postfix package from v3.8.0-r1 to v3.8.1-r0 (see links below)

https://web.archive.org/web/20230609193435/https://www.postfix.org/announcements/postfix-3.8.1.html

https://web.archive.org/web/20230609193412/https://git.alpinelinux.org/aports/commit/main/postfix?h=3.18-stable&id=8b279a5384f73e1d75583a8a61ffadad42662bec

Dockerfile

@@ -2,7 +2,7 @@ FROM docker.io/alpine:3.18.0
 
 # http://www.postfix.org/announcements.html
 # https://git.alpinelinux.org/aports/log/main/postfix?h=3.18-stable
-ARG POSTFIX_PACKAGE_VERSION=3.8.0-r1
+ARG POSTFIX_PACKAGE_VERSION=3.8.1-r0
 # http://www.postfix.org/postconf.5.html#tls_append_default_CA
 # https://www.mozilla.org/en-US/about/governance/policies/security-group/certs/
 # https://gitlab.alpinelinux.org/alpine/ca-certificates/-/commits/master