upgrade postfix package from v3.7.2-r0 to v3.7.3-r0

https://web.archive.org/web/20221014174231/https://www.postfix.org/announcements/postfix-3.7.3.html
https://web.archive.org/web/20221014174207/https://git.alpinelinux.org/aports/commit/main/postfix?h=3.16-stable&id=59bf173b777ec90191bcaf7e9d7b372c1437ffe8

Dockerfile

@@ -1,6 +1,6 @@
 FROM alpine:3.16.2
 
-ARG POSTFIX_PACKAGE_VERSION=3.7.2-r0
+ARG POSTFIX_PACKAGE_VERSION=3.7.3-r0
 # http://www.postfix.org/postconf.5.html#tls_append_default_CA
 # https://www.mozilla.org/en-US/about/governance/policies/security-group/certs/
 ARG MOZILLA_CA_CERT_STORE_PACKAGE_VERSION=20220614-r0