Creating a Ruby Gem Scaffold with Docker and PostgreSQL

March 25, 2025

Introduction

When building a new Ruby gem, setting up a consistent and isolated development environment is crucial. In this article, I’ll walk you through how I set up a Docker-based scaffold for a Ruby gem, ensuring a clean and reproducible setup with PostgreSQL integration.

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Why Use Docker for Gem Development?

Docker allows you to:

• Keep dependencies isolated
• Ensure consistent environments across machines
• Avoid conflicts with system-wide Ruby versions and libraries

Setting Up the Environment

To containerize the development environment, we use a Dockerfile and docker-compose.yml.

Dockerfile

This Dockerfile sets up a lightweight Alpine-based Ruby environment:

FROM ruby:3.4.2-alpine

RUN apk update && apk upgrade && apk add --no-cache \
    bash build-base libxml2-dev libxslt-dev postgresql-client postgresql-dev \
    npm yarn tzdata shared-mime-info git imagemagick imagemagick-dev imagemagick-libs \
    curl yaml-dev && \
    gem install nokogiri

WORKDIR /app
COPY . .

RUN bundle install --jobs=10
RUN yarn install --modules-folder /node_modules

ENV PATH /usr/local/bundle/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/node_modules/.bin 

docker-compose.yml

The docker-compose.yml file orchestrates the PostgreSQL database and the Ruby environment:

services:
  db:
    container_name: gem-rails.db
    image: postgres:latest
    environment:
      - POSTGRES_USER=psql
      - POSTGRES_PASSWORD=psql
    ports:
      - "5432:5432"
    volumes:
      - ${PWD}/local_development/init-database:/docker-entrypoint-initdb.d/
      - db_store:/var/lib/postgresql/data

  rails:
    build:
      dockerfile: Dockerfile
    container_name: gem-rails.rails
    depends_on:
      - db
    environment:
      - BASE_URL=http://localhost:3000
      - POSTGRES_USER=psql
      - POSTGRES_PASSWORD=psql
      - RAILS_ENV=development

      - BUNDLE_GEM__TEST=rspec
      - BUNDLE_GEM__CI=github
      - BUNDLE_GEM__MIT=true
      - BUNDLE_GEM__COC=true
      - BUNDLE_GEM__CHANGELOG=true
      - BUNDLE_GEM__LINTER=false
    ports:
      - "3000:3000"
    tty: true
    volumes:
      - ${PWD}:/app
      - ${PWD}/.root:/root
    platform: linux/amd64

volumes:
  db_store: 

Creating the Gem Scaffold

Once the environment is ready, you can generate the gem scaffold by running the following commands:

# git clone https://github.com/ggerman/create_gem
# cd create_gem
# docker-compose -f docker-compose.yml run --rm rails bash
# bundler gem ai --test=rspec 

This will create a gem scaffold named ai with RSpec for testing.

Alternatively, you can use the create_gem repository to streamline the gem creation process. This tool provides a structured and efficient way to scaffold a new Ruby gem with best practices built-in.

Conclusion

By using Docker, we ensure that our gem development environment remains stable, version-controlled, and easily replicable. This setup is especially useful when working with teams or switching between projects without affecting your local system.

Let me know your thoughts or if you have suggestions for improving this setup!