Bump Ruby from 4.0.1 to 4.0.3

Patch release within 4.0.x — security and bug fixes only, no source
changes required. Updated both .ruby-version and the Dockerfile ARG
they're explicitly told to keep in sync.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.ruby-version

@@ -1 +1 @@
-4.0.1
+4.0.3

Dockerfile

@@ -8,7 +8,7 @@
 # For a containerized dev environment, see Dev Containers: https://guides.rubyonrails.org/getting_started_with_devcontainer.html
 
 # Make sure RUBY_VERSION matches the Ruby version in .ruby-version
-ARG RUBY_VERSION=4.0.1
+ARG RUBY_VERSION=4.0.3
 FROM docker.io/library/ruby:$RUBY_VERSION-slim AS base
 
 LABEL org.opencontainers.image.source=https://github.com/dkam/clinch