Enable Docker BuildKit for faster image builds

Enables BuildKit in both CI and local development:
- Set DOCKER_BUILDKIT=1 in GitHub Actions workflow
- Set DOCKER_BUILDKIT=1 in Rakefile for local runs

Benefits:
- Faster image builds with improved layer caching
- Better build output with progress information
- Parallel build stage execution
- More efficient use of build cache across runs
- Better handling of multi-stage builds

BuildKit is Docker's next-generation build system and is now
the default in Docker Desktop and newer Docker versions.

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

Author: chorrell

.github/workflows/ruby.yml

@@ -15,4 +15,6 @@ jobs:
         ruby-version: 3.4
         bundler-cache: true
     - name: Run test
+      env:
+        DOCKER_BUILDKIT: 1
       run: bundle exec rake

Rakefile

@@ -1,6 +1,9 @@
 require 'rake'
 require 'rspec/core/rake_task'
 
+# Enable Docker BuildKit for faster builds with better caching
+ENV['DOCKER_BUILDKIT'] = '1'
+
 task :spec    => 'spec:all'
 task :default => :spec