Update default base image to 22.04

Brings us in line with GCC, and that means
we can run GCCs built by the gcc-builder newer than 14.

See also #78

Dockerfile

@@ -1,4 +1,4 @@
-ARG image=20.04
+ARG image=22.04
 FROM ubuntu:${image}
 LABEL maintainer="Matt Godbolt <[email protected]>"
 ARG image # https://docs.docker.com/engine/reference/builder/#understand-how-arg-and-from-interact