upgrade alpine base image v3.13.0->3.13.1 including upgrade of postfix v3.5.8-r0->v3.5.9-r0 (changes below)
> Runtime detection of DNSSEC support
> [...]
> With this update, the Postfix build system will no longer automatically disable DNSSEC support when it determines that Postfix will use libc-musl.
http://www.postfix.org/announcements/postfix-3.5.9.html
https://git.alpinelinux.org/aports/commit/main/postfix?id=1f69236b64bb0c5bd19989d2a39e7612e0ef0872