Use Ubuntu 20.04 instead of 22.04 as base image

22.04 already ships OpenSSL 3, which is not supported by Mumble 1.4. So
at least for the time being, we stick with 20.04 a while longer.

Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:22.04 as base
+FROM ubuntu:20.04 as base
 
 ARG DEBIAN_FRONTEND=noninteractive
 RUN apt-get update && apt-get install --no-install-recommends -y \