How to fix Gitea act_runner Docker error: Invalid input, please re-run act command: token is empty

Problem

While starting up your Gitea act_runner docker container, you repeatedly see the following error message

level=error msg="Invalid input, please re-run act command." error="token is empty"

Solution

In your docker-compose.yml, you need to set the following environment variables in order to allow the runner to register:

services:
  runner:
    # [...]
    environment:
      GITEA_INSTANCE_URL: https://gitea.my-domain.com
      GITEA_RUNNER_REGISTRATION_TOKEN: n73HEsTX71NMoVFJoMVSCEpin2IeihDnJZHELnNs
      GITEA_RUNNER_NAME: myrunner

After that, restart the docker-compose service.