In this tutorial, we will learn how to create a new rails app using docker. To speak on a broader sense, we will not have rails installed in our host machine rather we run new rails app within a docker container, however, we don’t need to get into docker container everytime we run the app except for the first time or for debugging purposes. Let’s get into the tutorial to understand how we reach the mentioned goals.

Step 1: Create project directory

Create your project directory railsapp. Inside the new folder, create a Dockerfile and pull ruby image and run required installations. On top of it, install rails.

    FROM ruby:3.1
    RUN apt update -qq && apt install -y nodejs postgresql-client
    RUN gem install rails -v '7.0.3.1'

Let’s build the docker image with the command docker build -t railsapp:latest .

Step 2: Create new rails app with rails-new

Now let’s run the docker and bash within the docker with the below command,

    docker run --rm -it -v $(pwd):/project railsapp:latest /bin/bash

Inside the docker, run the below commands to create a new rails app with a postgresql database. Once the new rails app is created, move the folder railsapp within our project directory. Finally exit the docker terminal.

    rails new railsapp --api --database=postgresql --skip-bundle
    mv railsapp/{*,.*} /project/
    exit

Step 3: Update Dockerfile from the host

Docker, by default creates the project folder with the root permissions. In order to change those permissions, run sudo chown -R $USER:$USER . within the project folder.

Let’s update the config/database.yml file

    Add the following in config/database.yml file
    default: &default
    # other stuff
    # adapter: ...
    # encoding: ...
    # pool: ...
    host: db
    username: postgres
    password: password

Now, update the Dockerfile with the below lines.

    FROM ruby:3.1
    RUN apt update -qq && apt install -y nodejs postgresql-client
    RUN gem install rails -v '7.0.3.1'

    ARG APP=/railsapp
    WORKDIR ${APP}
    ADD Gemfile ${APP}/
    RUN bundle install

Let’s breakdown the updated lines. The app folder is set to the argument APP and make it the working directory WORKDIR. Now, add the gemfile within the app folder and finally run the bundle to install.

Step 4: Create docker compose file

Create a docker-compose.yml file and copy the below lines of code.

    version: "2.2"
    services:
    db:
        image: postgres
    web:
        build: .
        command: bundle exec rails s -p 3000 -b '0.0.0.0'
        ports:
        - "3000:3000"
        depends_on:
        - db
        volumes:
        - ./:/railsapp

We have two services, db and web. For the db service, pull postgres image from the docker hub. For the web service, mention the required port and dependencies.

Step 5: Run docker compose

Run the below commands

    docker compose build
    docker compose up

While the docker compose is up and running, open a new terminal and run create the db

    docker compose run web rails db:create

Now, you can view your app from http://localhost:3000

Adding env file in rails app

It is a good practice to keep the configuration in a separate file inside the codebase. Fortunately, the variables specified in docker-compose.yml file, without no extra setup, will be subsituted with the variables specified inside the .env file (if they are not present in the environment variables already).

Steps to add .env file

1. Add a new line .env at the end of .gitignore file if it is not present already. This .env file which we will create in the next step contains all the secrets and hence is added to .gitignore before creation!

2. Add a new .env file with the following contents:

    RAILS_APPNAME=railsapp
    RAILS_DEMO_POSTGRES_USERNAME=railsapp
    RAILS_DEMO_POSTGRES_PASSWORD=password
    RAILS_DEMO_POSTGRES_HOST=db
   

3. postgres docker image allows creating specific roles with the specified password using the entrypoint script. Add under db in docker-compose.yml,

    services:
        db:
            environment:
                - POSTGRES_USER=${RAILS_DEMO_POSTGRES_USERNAME}
                - POSTGRES_PASSWORD=${RAILS_DEMO_POSTGRES_PASSWORD}
   

4. All the environment variables inside .env file are required for our rails app and so, under web in docker-compose.yml add,

    services:
        web:
            env_file:
                - .env
   

5. Replace the hardcoded variables previously added in the database.yml file

    default: &default
    # other stuff
    # adapter: ...
    # encoding: ...
    # pool: ...
    host: <%= ENV["RAILS_DEMO_POSTGRES_HOST"] %>
    username: <%= ENV["RAILS_DEMO_POSTGRES_USERNAME"] %>
    password: <%= ENV["RAILS_DEMO_POSTGRES_PASSWORD"] %>