The first step to specify the API tokens in an environment file named .env. The two required keys are BUTTER_TOKEN and MAILCHIMP_API_TOKEN, which have to be declared such as KEY=VALUE. When it's done you just have to run docker-compose up and the site starts in local on the port 3000.

BUTTER_TOKEN you will found in the settings tab after you logged in: https://buttercms.com/settings/.

MAILCHIMP_API_TOKEN you need to logged in to the MailChimp and navigate to the Account > Extras > API keys section.

MAILCHIMP_LIST_ID you will need to logged in to the MailChimp and go to the Lists and select the Lists > Settings > List name and defaults and you will found there the List ID block with you listing ID.