Add Ruby with Bundler (closes #8)

Install ruby, ruby-dev and bundler from apt. ruby-dev keeps native
gems buildable, matching what jekyllsitebuilder needs.

Model: deepseek-v4-pro

bin/test

@@ -30,6 +30,8 @@ docker run --rm "$img" bash -ec '
     vips -l | grep -i heif > /dev/null
     magick --version
     gs --version
+    ruby --version
+    bundle --version
 '
 
 echo "== $img: runit service lifecycle"