Steps to reproduce:

1. Create a new rails app

echo 'gem install rails && rails new . --skip-webpack-install --database=postgresql' | docker run -i --rm -v $PWD:/app -w /app ruby sh

2. Whitelist *.local.on-acorn.io in config.ru

Rails.application.configure do
    config.hosts << "*.local.on-acorn.io"
end

3. Enable environment variables for Postgres in config/database.yml

default: &default
  #...
  # add these
  host: <%= ENV.fetch("DATABASE_HOST") { "db" } %>
  database: <%= ENV.fetch("DATABASE_NAME") { "rails-example" } %>
  username: <%= ENV.fetch("DATABASE_USERNAME") { "rails-example" } %>
  password: <%= ENV.fetch("DATABASE_PASSWORD") { "rails-example" } %>

#...
test:
  <<: *default
  # remove existing

#...
production:
  <<: *default
  # remove existing

4. Add Acornfile, Dockerfile and entrypoint.sh

5. Ensure you can connect to your Docker Desktop with kubectl

6. Run acorn install

7. Run acorn run . or acorn run -i . for development mode

8. Add the *.local.on-acorn.io domain to your /etc/hosts file. For example:

127.0.0.1       server-holy-water-ab5e9d1e.local.on-acorn.io