fix(ci): remove DOCKER_TLS_CERTDIR to fix DinD connection failure

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.

.gitlab-ci.yml

@@ -40,7 +40,6 @@ variables:
   IMAGE: $CI_REGISTRY_IMAGE
   GIT_STRATEGY: clone
   DOCKER_DRIVER: overlay2
-  DOCKER_TLS_CERTDIR: "/certs"
   DOCKER_HOST: tcp://localhost:2375    
 
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