If your self-hosted GitLab server is using a self-signed certificate for https, it might be possible that you get an error during the registration of a GitLab Runner:
ERROR: Registering runner... failed runner=HbU25D-y status=couldn't execute POST against https://gitlab.example.com/api/v4/runners: Post https://gitlab.example.com/api/v4/runners: x509: certificate signed by unknown authority PANIC: Failed to register the runner. You may be having network problems.
To solve the problem, you have to provide the full chain certificate
*.pem used by your GitLab Server:
gitlab-runner register --tls-ca-file /path/to/fullchain.pem
In my case, the valid certificate could be found on the GitLab server in
/etc/gitlab/trusted-certs/fullchain.pem. This one was copied to the GitLab Runner server and used in the command above.
As I did not set up the server on my own, I do not know if this is the default path and filename of a certificate signed by Let’s Encrypt. But in my case, this one worked to register the runner.