Add Ruby 2.7 Dockerfile for Rails 5.1-6.0 compatibility

Add Dockerfile.ruby2.7 to support older Rails versions that are not
compatible with Ruby 3.2. Based on CI matrix compatibility:

Ruby 2.7 supports:
- Rails 5.1.7 ✅
- Rails 5.2.8.1 ✅
- Rails 6.0.6 ✅
- Rails 6.1.7.10 (also works with Ruby 3.2)
- Rails 7.0.10 (also works with Ruby 3.2)

Ruby 3.2 (existing Dockerfile):
- Rails 6.1.7.10 ✅
- Rails 7.0.10 ✅
- Rails 7.1.6 ✅
- Rails 7.2.3 ✅
- Rails 8.0.4 ✅
- Rails 8.1.2 ✅

Changes:
- Add Dockerfile.ruby2.7 with Ruby 2.7 base image
- Update docker-compose.yml to use Ruby 2.7 for Rails 5.1, 5.2, 6.0
- Add bundle-cache-ruby27 volume for separate gem caching
- Keep Ruby 3.2 for Rails 6.1-8.1

This ensures all Rails versions in the CI matrix can be tested locally
with the appropriate Ruby version.

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

Author: takaokouji

Dockerfile.ruby2.7

@@ -0,0 +1,22 @@
+# Dockerfile for testing jsonapi-resources with Rails 5.1-7.0 (Ruby 2.7)
+
+FROM ruby:2.7
+
+# 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

@@ -1,5 +1,5 @@
 services:
-  # Base service definition
+  # Base service definition for Ruby 3.2 (Rails 6.1-8.1)
   test-base: &test-base
     build:
       context: .
@@ -11,25 +11,37 @@ services:
     stdin_open: true
     tty: true
 
+  # Base service definition for Ruby 2.7 (Rails 5.1-7.0)
+  test-base-ruby27: &test-base-ruby27
+    build:
+      context: .
+      dockerfile: Dockerfile.ruby2.7
+    volumes:
+      - .:/app
+      - bundle-cache-ruby27:/usr/local/bundle
+    working_dir: /app
+    stdin_open: true
+    tty: true
+
   # Rails 5.1.7
   rails-5.1:
-    <<: *test-base
+    <<: *test-base-ruby27
     container_name: jsonapi-rails-5.1
     environment:
       - RAILS_VERSION=5.1.7
     command: bash -c "rm -f Gemfile.lock && bundle install && bundle exec rake test"
 
   # Rails 5.2.8.1
   rails-5.2:
-    <<: *test-base
+    <<: *test-base-ruby27
     container_name: jsonapi-rails-5.2
     environment:
       - RAILS_VERSION=5.2.8.1
     command: bash -c "rm -f Gemfile.lock && bundle install && bundle exec rake test"
 
   # Rails 6.0.6
   rails-6.0:
-    <<: *test-base
+    <<: *test-base-ruby27
     container_name: jsonapi-rails-6.0
     environment:
       - RAILS_VERSION=6.0.6
@@ -93,3 +105,4 @@ services:
 
 volumes:
   bundle-cache:
+  bundle-cache-ruby27: