Login to server by ssh as other user than ROOT

I tried set CI/CD in .gitlab-ci.yml, but I can't resolve problem with permission to the private key. My config

  - deploy

deploy_staging:
  stage: deploy
  before_script:
    - 'which ssh-agent || ( apt-get update -y && apt-get install openssh-client -y )'
    - eval $(ssh-agent -s)
    - mkdir -p ~/.ssh
    - chmod 400 ~/.ssh
    - echo "$SSH_PRIVATE_KEY" | ssh-add - > ~/.ssh/id_rsa
    - '[[ -f /.dockerenv ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" > ~/.ssh/config'
  script:
   - ssh-add <(echo "$SSH_PRIVATE_KEY")
   - ssh -p22 xxx@xxx
  environment:
    name: staging
    url :xxx

pipline does not pass and return an error :

 Warning: Permanently added 's13.zenbox.pl,2.57.137.13' (ECDSA) to the list of known hosts.
 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
 @         WARNING: UNPROTECTED PRIVATE KEY FILE!          @
 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
 Permissions 0644 for '/root/.ssh/id_rsa' are too open.
 It is required that your private key files are NOT accessible by others.
 This private key will be ignored.
 Load key "/root/.ssh/id_rsa": bad permissions
 Permission denied, please try again.
 Permission denied, please try again.
 xxx@xxx: Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password).

as you can see, ssh tries get access to /root/.ssh , but my application is developed on shared hosting and I don't access to root user. I think that it can be relating to the docker executor. I was reading that default use user is root. Can I change it?