Fix bundler version for Ruby 2.7 compatibility

Ruby 2.7 requires bundler < 2.5. Install bundler 2.4.22 specifically
to avoid compatibility issues.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: takaokouji

Dockerfile.ruby2.7

@@ -15,8 +15,8 @@ WORKDIR /app
 COPY Gemfile jsonapi-resources.gemspec ./
 COPY lib/jsonapi/resources/version.rb ./lib/jsonapi/resources/
 
-# Install bundler
-RUN gem install bundler
+# Install bundler (Ruby 2.7 requires bundler < 2.5)
+RUN gem install bundler -v 2.4.22
 
 # Note: bundle install will happen at runtime with specific RAILS_VERSION
 # This allows testing multiple Rails versions without rebuilding the image