Create bucket

s3cmd mb s3://artifacts

Configure GitLab

/etc/gitlab/gitlab.rb
...
nginx["client_max_body_size"] = "1024m"
gitlab_rails["artifacts_enabled"] = true
gitlab_rails["artifacts_object_store_enabled"] = true
gitlab_rails["artifacts_object_store_remote_directory"] = "artifacts"
gitlab_rails["artifacts_object_store_connection"] = {
  "provider" => "AWS",
  "region" => "us-west-1",
  "aws_access_key_id" => "KEY1234",
  "aws_secret_access_key" => "PASS1234",
  "endpoint" => "https://s3.example.com"
}

gitlab-ctl reconfigure

Links
https://docs.gitlab.com/ee/ci/yaml/#artifacts
https://docs.gitlab.com/ee/administration/job_artifacts.html#using-object-storage