Since Gitlab 11.7, a variable containing the URL to the gitlab API v4 is automatically set by gitlab: CI_API_V4_URL.

reviewdog could use this instead of GITLAB_API, perhaps as a fall-back mechanism (try GITLAB_API first, fall back to CI_API_V4_URL if the former is not set) to not break existing configurations.

I have not checked the code but the change seems pretty straightforward, so I could probably work on a PR for this if you are interested.