Update gitlab_ci.md (#6063)

Clarity around the port number for unencrypted docker host connections, as most gitlab DinD examples recommend 2736 without explaining why.

Author: BenManifold

docs/supported_docker_environment/continuous_integration/gitlab_ci.md

@@ -46,7 +46,7 @@ services:
     command: ["--tls=false"]
 
 variables:
-  # Instruct Testcontainers to use the daemon of DinD.
+  # Instruct Testcontainers to use the daemon of DinD, use port 2735 for non-tls connections.
   DOCKER_HOST: "tcp://docker:2375"
   # Instruct Docker not to start over TLS.
   DOCKER_TLS_CERTDIR: ""