upgrade dovecot packages from v2.3.20-r6 to v2.3.20-r7 (adds support for zstd compression)

https://web.archive.org/web/20230318090513/https://git.alpinelinux.org/aports/commit/main/dovecot?h=3.17-stable&id=db057b7badb30307a5c66dead3049008de2c2c07

Dockerfile

@@ -1,6 +1,6 @@
 FROM docker.io/alpine:3.17.2
 
-ARG DOVECOT_PACKAGE_VERSION=2.3.20-r6
+ARG DOVECOT_PACKAGE_VERSION=2.3.20-r7
 ARG SOCAT_PACKAGE_VERSION=1.7.4.4-r0
 RUN apk add --no-cache \
         dovecot-lmtpd=$DOVECOT_PACKAGE_VERSION \