Merge #1730

1730: Use alpine 3.13 to fix CVE-2020-25275 and CVE-2020-24386 r=mergify[bot] a=micw

## What type of PR?

bug-fix

## What does this PR do?

Upgrade dovecot alpine to 3.13 to fix CVEs in dovecot

### Related issue(s)

- #1720

## Prerequistes
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.

- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/guide.html#changelog) entry file.


Co-authored-by: Michael Wyraz <michael@wyraz.de>

core/dovecot/Dockerfile

@@ -1,4 +1,4 @@
-ARG DISTRO=alpine:3.12
+ARG DISTRO=alpine:3.13
 FROM $DISTRO as builder
 WORKDIR /tmp
 RUN apk add git build-base automake autoconf libtool dovecot-dev xapian-core-dev icu-dev

towncrier/newsfragments/1720.bugfix

@@ -0,0 +1,2 @@
+Fix CVE-2020-25275 and CVE-2020-24386 by using alpine 3.13 for
+dovecot which contains a fixed dovecot version.