CI runner can't reach the Docker daemon

Your GitHub Actions self-hosted runner fails with 'Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?' Walk through your diagnosis. Expected reasoning: confirm dockerd is running (systemctl status docker), confirm the runner's user is in the docker group or has socket permissions, check DOCKER_HOST isn't pointing somewhere stale, and check the socket file actually exists (it may be in a container without the socket mounted, in which case the build needs docker-in-docker or socket-mounting with awareness of the security trade-off).