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`