Gitlab deploy to FortRabbit

I am following the tutorial to deploy Laravel application with GitLab CI/CD on DigitalOcean server (https://docs.gitlab.com/ee/ci/examples/laravel_with_gitlab_and_envoy/) and it works great.

I am trying to do the deployment to FortRabbit and kinda stuck

This is my .gitlab-ci.yml

image: registry.gitlab.com/{{username}}/laravel57

services:
  - mysql:5.7

variables:
  MYSQL_DATABASE: homestead
  MYSQL_ROOT_PASSWORD: secret
  DB_HOST: mysql
  DB_USERNAME: root

stages:
  - test
  - deploy

unit_test:
  stage: test
  script:
    - cp .env.example .env
    - composer install
    - php artisan key:generate
    - php artisan migrate
    - vendor/bin/phpunit

deploy_production:
  stage: deploy
  script:
    - 'which ssh-agent || ( apt-get update -y && apt-get install openssh-client -y )'
    - eval $(ssh-agent -s)
    - ssh-add <(echo "$SSH_PRIVATE_KEY_FROM_HOMESTEAD")
    - mkdir -p ~/.ssh
    - '[[ -f /.dockerenv ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" > ~/.ssh/config'
    - git clone git@gitlab.com:{{username}}/laravel57.git
    - cd laravel57
    - git remote add fortrabbit {{app-name}}@deploy.us1.frbit.com:{{app-name}}.git
    - git push -u fortrabbit master
  
  environment:
    name: production
    url: http://192.168.1.1
  when: manual
  only:
    - master

What I got is

Cloning into 'laravel57'...
Warning: Permanently added 'gitlab.com,xx.xxx.xx.xxx' (ECDSA) to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
ERROR: Job failed: exit code 1

Any suggestions?