Add Docker services for Rails 5.1, 5.2, and 6.0

Add Docker Compose services for older Rails versions to enable
comprehensive testing across the full compatibility matrix:

Services added:
- rails-5.1: Rails 5.1.7
- rails-5.2: Rails 5.2.8.1
- rails-6.0: Rails 6.0.6

Usage:
  docker-compose run rails-5.1
  docker-compose run rails-5.2
  docker-compose run rails-6.0

This completes the Docker setup for all Rails versions supported
in the CI matrix (5.1.7 through 8.1.2).

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

Author: takaokouji

docker-compose.yml

@@ -11,6 +11,30 @@ services:
     stdin_open: true
     tty: true
 
+  # Rails 5.1.7
+  rails-5.1:
+    <<: *test-base
+    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
+    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
+    container_name: jsonapi-rails-6.0
+    environment:
+      - RAILS_VERSION=6.0.6
+    command: bash -c "rm -f Gemfile.lock && bundle install && bundle exec rake test"
+
   # Rails 6.1.7.10
   rails-6.1:
     <<: *test-base