Dockerfile: Switch to bullseye

But allow users to change it via a build-arg

docker/Dockerfile

@@ -1,4 +1,7 @@
-FROM debian:buster-slim
+ARG DEBIAN_RELEASE=bullseye
+FROM debian:${DEBIAN_RELEASE}-slim
+ARG DEBIAN_RELEASE=bullseye
+ENV DEBIAN_RELEASE=$DEBIAN_RELEASE
 
 RUN	apt-get update && apt-get install -y --no-install-recommends \
 		ca-certificates \
@@ -46,7 +49,7 @@ lwJWDec6PUe835YwqrARXtPaNA==                                            \n\
 	&& rm -rf "$GNUPGHOME" \
 	&& apt-key list > /dev/null
 
-RUN	echo "deb [signed-by=/etc/apt/trusted.gpg.d/rspamd.gpg] https://rspamd.com/apt-stable/ buster main" > /etc/apt/sources.list.d/rspamd.list
+RUN	echo "deb [signed-by=/etc/apt/trusted.gpg.d/rspamd.gpg] https://rspamd.com/apt-stable/ $DEBIAN_RELEASE main" > /etc/apt/sources.list.d/rspamd.list
 
 RUN	apt-get update \
 	&& apt-get install -y rspamd \