Update to debian bullseye

Update to debian bullseye

diff --git a/Dockerfile b/Dockerfile
index ee0b222..906cde1 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM ruby:2.7-slim-buster
+FROM ruby:2.7-slim-bullseye
 
 MAINTAINER Michael Brown "michael.brown@discourse.org"
 

GitHub sha: d3adefccae077b094651edc624f132bbd25af044