Update to debian bullseye

Update to debian bullseye

diff --git a/Dockerfile b/Dockerfile
index 7e6be7c..a89dfda 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM debian:buster-slim
+FROM debian:bullseye-slim
 
 RUN DEBIAN_FRONTEND=noninteractive apt update \
 	&& DEBIAN_FRONTEND=noninteractive apt install -y --no-install-recommends curl perl postfix ruby socat \

GitHub sha: 675928405f9eec22763d32d28dc1486b2f926517