Move Sidekiq to its own container

[JulienSansot]

Sidekiq was introduced to fix issue #98

At the moment the Sidekiq process is run in the same container as the web server with foreman and this Procfile:

web: bundle exec puma -C config/puma.rb -p 3002
worker: bundle exec sidekiq

Instead, we should have Sidekiq run in its own container. It makes more sense to have one process per container and we can control them individually. For example we can then scale them independently