upgrade alpine base image v3.13.4->v3.13.5 including postfix package upgrade from v3.5.9-r0->v3.5.10-r0 (see links below)
> fixes address null pointer bugs and a memory leak that have no potential for information loss or leaks, privilege escalation, or denial of service.
http://www.postfix.org/announcements/postfix-3.5.10.html
https://git.alpinelinux.org/aports/commit/main/postfix?id=f5936eac1df0f83f7889db819188a593ea005518