Switch from ruby-slim to ruby image

This is required in order to have -dev packages available during our
build stage. Apparently, this should have never worked and the upstream
image has just been cleaned up to align with documentation.

See https://github.com/docker-library/ruby/pull/497

Closes !25

Dockerfile

@@ -3,7 +3,7 @@
 # Make sure RUBY_VERSION matches the Ruby version in .ruby-version and Gemfile
 # renovate: datasource=docker depName=registry.docker.com/library/ruby
 ARG RUBY_VERSION=3.4.2
-FROM registry.docker.com/library/ruby:$RUBY_VERSION-slim AS base
+FROM registry.docker.com/library/ruby:$RUBY_VERSION AS base
 
 # Rails app lives here
 WORKDIR /rails