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

README.md

@@ -41,6 +41,8 @@ Image: `git.eeqj.de/sneak/baseimage`, built for `linux/amd64` and
 | libheif         | apt `libheif-dev`, `libheif-plugin-aomenc` | HEIF/AVIF codec   |
 | ImageMagick     | apt                            | `magick`                       |
 | Ghostscript     | apt                            | `gs`                           |
+| Ruby            | apt `ruby`, `ruby-dev`         | `/usr/bin/ruby`                |
+| Bundler         | apt `bundler`                  | `bundle` on `PATH`             |
 
 Plus the usual odds and ends: `git`, `curl`, `wget`, `jq`, `rsync`,
 `openssh-client`, `zip`/`unzip`, `xz-utils`, `bzip2`, and the `-dev`

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"

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)"
@@ -115,6 +116,8 @@ vips --version
 vips -l | grep -i heif > /dev/null
 magick --version
 gs --version
+ruby --version
+bundle --version
 hugo version
 node --version
 npm --version