melange.yaml

package:
  name: Sinatra
  version: 0.1.0
  description: Sinatra melange demo
  target-architecture:
    - aarch64
    - x86_64
  copyright:
    - license: Apache-2.0
      paths:
        - "example2/*"
  dependencies:
    runtime:
      - wolfi-baselayout
      - ca-certificates-bundle
      - ruby3.2-bundler

environment:
  contents:
    repositories:
      - https://packages.wolfi.dev/os
    packages:
      - wolfi-baselayout
      - ca-certificates-bundle
      - busybox
      - wget
      - make
      - gcc
      - linux-headers
      - ruby3.2-bundler

pipeline:
  - name: Build sinatra application
    runs: |
       mkdir -p /usr/lib/ruby
       cd /usr/lib/ruby/
       wget https://cache.ruby-lang.org/pub/ruby/3.2/ruby-3.2.1.tar.gz
       tar xfz ruby-3.2.1.tar.gz
       ln -s ./ruby-3.2.1/include ./include
       cd -
       cd "${MINICLI_HOME}"
       cd example2
       bundle install

You can add ruby-3.2.1 as a dep, there is  ruby-3.2-dev in there as well. https://github.com/wolfi-dev/os/blob/main/ruby-3.2.yaml