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

build.sh

@@ -30,7 +30,8 @@ apt-get -y install --no-install-recommends \
     tzdata \
     zstd sqlite3 \
     libvips-dev libvips-tools libheif-dev libheif-plugin-aomenc \
-    imagemagick ghostscript
+    imagemagick ghostscript \
+    ruby ruby-dev bundler
 
 # --- Go (latest) -------------------------------------------------------------
 gover="$(curl -fsSL 'https://go.dev/VERSION?m=text' | head -n1)"
@@ -113,6 +114,8 @@ vips --version
 vips -l | grep -i heif > /dev/null
 magick --version
 gs --version
+ruby --version
+bundle --version
 hugo version
 node --version
 npm --version