Use a Gemfile in Dockerfile

[harigopal]

I don't see a reason for why we're manually installing gems inside the Dockerfile. Shouldn't we be using a Gemfile instead and get the
added reliability from a Gemfile.lock.

This could be the Gemfile:

source 'https://rubygems.org'

ruby '3.2.3'

# A Ruby library for declaring, composing and executing GraphQL queries.
gem 'graphql-client', '~> 0.20.0'

# A Ruby gem for the OpenAI API.
gem 'ruby-openai', '~> 6.3'

group :development do
  gem 'pry', '>= 1.0.0.pre1'
  gem 'dotenv', '~> 3.1'
end