alpine v3.10 -> v3.11; pin dovecot package version

CHANGELOG.md

@@ -5,6 +5,12 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Changed
+- upgrade base image: alpine v3.10 -> v3.11
+
+### Fixed
+- pin dovecot package version
+  ( https://github.com/hadolint/hadolint/wiki/DL3018 )
 
 ## [0.1.1] - 2019-08-25
 - mount dir `/etc/ssl/dovecot` providing SSL key & cert

Dockerfile

@@ -1,6 +1,7 @@
-FROM alpine:3.10
+FROM alpine:3.11
 
-RUN apk add --no-cache dovecot
+ARG DOVECOT_PACKAGE_VERSION=2.3.9.3-r0
+RUN apk add --no-cache dovecot=$DOVECOT_PACKAGE_VERSION
 
 VOLUME /etc/ssl/dovecot