DOCKER_TLS_CERTDIR forced the docker:28.4.0-dind service to listen on
port 2376 with TLS, but DOCKER_HOST pointed to the non-TLS port 2375.
This caused the docker client to loop forever with:
Cannot connect to the Docker daemon at tcp://localhost:2375.
Removing DOCKER_TLS_CERTDIR lets the daemon listen on 2375 again,
matching DOCKER_HOST, restoring docker-in-docker connectivity.