Change Rails 6.1-8.1 to use Ruby 3.1.5

Replace Ruby 3.2 with Ruby 3.1.5 for Rails 6.1-8.1 testing.
This change will be validated by running tests on each version
sequentially.

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

Author: takaokouji

Dockerfile.ruby3.1

@@ -0,0 +1,22 @@
+# Dockerfile for testing jsonapi-resources with Rails 6.1-8.1 (Ruby 3.1.5)
+
+FROM ruby:3.1.5
+
+# Install dependencies
+RUN apt-get update -qq && \
+    apt-get install -y build-essential libpq-dev nodejs && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
+
+# Set working directory
+WORKDIR /app
+
+# Copy Gemfile and gemspec
+COPY Gemfile jsonapi-resources.gemspec ./
+COPY lib/jsonapi/resources/version.rb ./lib/jsonapi/resources/
+
+# Install bundler
+RUN gem install bundler
+
+# Note: bundle install will happen at runtime with specific RAILS_VERSION
+# This allows testing multiple Rails versions without rebuilding the image

docker-compose.yml

@@ -1,17 +1,17 @@
 services:
-  # Base service definition for Ruby 3.2 (Rails 6.1-8.1)
+  # Base service definition for Ruby 3.1.5 (Rails 6.1-8.1)
   test-base: &test-base
     build:
       context: .
-      dockerfile: Dockerfile
+      dockerfile: Dockerfile.ruby3.1
     volumes:
       - .:/app
       - bundle-cache:/usr/local/bundle
     working_dir: /app
     stdin_open: true
     tty: true
 
-  # Base service definition for Ruby 2.7 (Rails 5.1-7.0)
+  # Base service definition for Ruby 2.7 (Rails 5.1-6.0)
   test-base-ruby27: &test-base-ruby27
     build:
       context: .