Upgrade to bullseye, use registry cache

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

Dockerfile

@@ -1,4 +1,4 @@
-FROM debian:buster-slim
+FROM registry.fk.jochum.dev/docker_hub_cache/library/debian:bullseye-slim
 
 LABEL maintainer "René Jochum <rene@jochum.dev>"
 
@@ -21,7 +21,7 @@ RUN set -ex; \
     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
+    echo "deb https://repo.dovecot.org/ce-2.3-latest/debian/bullseye bullseye main" > /etc/apt/sources.list.d/dovecot.list
 
 RUN set -ex; \
     apt-get update --allow-releaseinfo-change && \