Failed to register Gitlab Runner (Docker): x509: certificate signed by unknown authority

Hi,

i'm trying to register a docker gitlab runner to my gitlab instance. The Gitlab instance is using a certificate issued by our internal CA. This CA ist trusted by the machine where docker is running on (A normal curl against the gitlab instance is working without an error). But although I'm getting this error message after entering the url, token etc. while registering:

ERROR: Registering runner... failed                 runner=WYvef3JP status=couldn't execute POST against https://mygitserver.local/api/v4/runners: Post https://mygitserver.local/api/v4/runners: x509: certificate signed by unknown authority
PANIC: Failed to register this runner. Perhaps you are having network problems

This is the register command I'm issuing: docker run --rm -t -i -v /srv/gitlab-runner/config:/etc/gitlab-runner --name gitlab-runner gitlab/gitlab-runner register

I also tried this command with appended tls-ca-file=/path/to/my/cachain.crt

Does anyone know how i get this to work?

Thanks in advance!