Set fully qualified reference name for base image (rails#47888)

Explicitly set the fully qualified reference name to avoid issues in
environments which set a different default registry.
The registry at registry.docker.com is dual stack-compatible.

Author: juliusrickert

railties/lib/rails/generators/rails/app/templates/Dockerfile.tt

@@ -2,7 +2,7 @@
 
 # Make sure RUBY_VERSION matches the Ruby version in .ruby-version and Gemfile
 ARG RUBY_VERSION=<%= Gem.ruby_version %>
-FROM ruby:$RUBY_VERSION-slim as base
+FROM registry.docker.com/library/ruby:$RUBY_VERSION-slim as base
 
 # Rails app lives here
 WORKDIR /rails