Use dovecot repo

Signed-off-by: René Jochum <rene@jochum.dev>

Dockerfile

@@ -2,6 +2,13 @@ FROM registry.jochum.dev/library/debian:buster-slim
 
 LABEL maintainer="rene@jochum.dev"
 
+RUN set -ex; \
+    apt-get update --allow-releaseinfo-change && \
+    apt-get install --no-install-recommends -y -o 'DPkg::Options::=--force-confold' -o 'DPkg::Options::=--force-confdef' gnupg apt-transport-https curl ca-certificates && \
+    curl https://repo.dovecot.org/DOVECOT-REPO-GPG | gpg --import && \
+    gpg --export ED409DA1 > /etc/apt/trusted.gpg.d/dovecot.gpg && \
+    echo "deb https://repo.dovecot.org/ce-2.3-latest/debian/buster buster main" > /etc/apt/sources.list.d/dovecot.list
+
 RUN set -ex; \
     apt-get update --allow-releaseinfo-change && \
     apt-get install --no-install-recommends -y -o 'DPkg::Options::=--force-confold' -o 'DPkg::Options::=--force-confdef' dovecot-imapd dovecot-pop3d dovecot-lmtpd dovecot-submissiond dovecot-managesieved dovecot-sieve dovecot-pgsql dovecot-mysql python3-minimal python3-jinja2 && \