Set Ruby version to 3.1.6 in Dockerfile

Our Rails intergration tests run on a single version of Ruby so we
default to a compatible version.

When running our Ruby version tests the default is overridden.

Dockerfile

@@ -1,5 +1,5 @@
 # We want to support older Rubies
-ARG RUBY_VERSION=2.7.8
+ARG RUBY_VERSION=3.1.6
 FROM registry.docker.com/library/ruby:$RUBY_VERSION-slim as base
 
 # Rails app lives here