Update dovecot/Dockerfile, dovecot/Makefile, .gitlab-ci.yml files

dovecot/Dockerfile

@@ -0,0 +1,29 @@
+ARG DEBIAN_VERSION=20210408
+FROM docker.io/debian:bullseye-${DEBIAN_VERSION}-slim
+
+ENV container=docker
+ENV LC_ALL=C
+ENV DEBIAN_FRONTEND=noninteractive
+RUN apt-get -y update && apt-get -y install \
+    tini \
+    dovecot-core \
+    dovecot-imapd \
+    dovecot-lmtpd \
+    dovecot-ldap \
+    dovecot-managesieved \
+    dovecot-mysql \
+    dovecot-pgsql \
+    dovecot-sqlite \
+    dovecot-pop3d \
+    dovecot-sieve \
+    dovecot-solr \
+    dovecot-submissiond \
+    ca-certificates \
+    ssl-cert \
+    && rm -rf /etc/dovecot \
+    && mkdir /srv/mail && chown dovecot:dovecot /srv/mail \
+    && rm -rf /var/lib/apt/lists/*
+
+VOLUME [/etc/dovecot /srv/mail /var/mail]
+ENTRYPOINT ["/sbin/tini" "--"]
+CMD ["/usr/sbin/dovecot" "-F"]