Use host mode for docker network

In order to reduce the traffic of the secondary Ethernet device we need
to be able to directly use the physical device instead of the virtual
one. For now use the host mode and see later if we can revert it.

Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/tent/gitlab-runner.nix

@@ -43,6 +43,7 @@
         registrationFlags = [
           # Increase build log length to 64 MiB
           "--output-limit 65536"
+          "--docker-network-mode host"
         ];
         preBuildScript = pkgs.writeScript "setup-container" ''
           mkdir -p -m 0755 /nix/var/log/nix/drvs