upgrade dovecot packages from v2.3.14-r0 to v2.3.15-r0 (see links below)

https://github.com/dovecot/core/releases/tag/2.3.15
https://git.alpinelinux.org/aports/commit/main/dovecot?id=d25a27d17f2d3a15589e5c781009b3d10f581f59

Dockerfile

@@ -1,6 +1,6 @@
 FROM alpine:3.14.0
 
-ARG DOVECOT_PACKAGE_VERSION=2.3.14-r0
+ARG DOVECOT_PACKAGE_VERSION=2.3.15-r0
 ARG SOCAT_PACKAGE_VERSION=1.7.4.1-r1
 RUN apk add --no-cache \
         dovecot-lmtpd=$DOVECOT_PACKAGE_VERSION \