Use docker.io as the default registry for Ruby images

The `registry.docker.com` registry isn't documented and have a delay
when pulling images. The `docker.io` registry is the default registry
for Docker images and is the one used by the Docker CLI.

Author: rafaelfranca

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

@@ -8,7 +8,7 @@
 
 # Make sure RUBY_VERSION matches the Ruby version in .ruby-version
 ARG RUBY_VERSION=<%= gem_ruby_version %>
-FROM registry.docker.com/library/ruby:$RUBY_VERSION-slim as base
+FROM docker.io/library/ruby:$RUBY_VERSION-slim as base
 
 # Rails app lives here
 WORKDIR /rails

railties/test/fixtures/Dockerfile.test

@@ -2,7 +2,7 @@
 
 # Make sure RUBY_VERSION matches the Ruby version in .ruby-version and Gemfile
 ARG RUBY_VERSION=3.2.2
-FROM registry.docker.com/library/ruby:$RUBY_VERSION-slim as base
+FROM docker.io/library/ruby:$RUBY_VERSION-slim as base
 
 # Rails app lives here
 WORKDIR /rails