refactor: move vendor NOS Docker support into VendorDockerVM subclass

Extract the docker_exec console and GNS3_* prototype knobs (SKIP_INIT,
INTERFACE_NAMES, CONSOLE_CMD) from DockerVM into a VendorDockerVM subclass.
DockerVM is restored to its 3.1 baseline plus four small extension hooks
(_prepare_init_and_interface_env, _start_console_server,
_get_container_ifname, _cleanup_console_resources) that are pure
refactorings with zero behaviour change for existing nodes.

VendorDockerVM additionally replicates init.sh's volume persistence
(bind-mount /gns3volumes over the in-container path) via docker exec for
containers that skip init.sh, so vendor NOS config (e.g. /etc/opt/srlinux)
survives node stop/start.

The Docker manager selects VendorDockerVM when console_type == docker_exec;
all other nodes keep using DockerVM unchanged.

gns3server/compute/docker/__init__.py

@@ -32,6 +32,7 @@ from gns3server.config import Config
 from gns3server.utils.asyncio import locking
 from gns3server.compute.base_manager import BaseManager
 from gns3server.compute.docker.docker_vm import DockerVM
+from gns3server.compute.docker.vendor_docker_vm import VendorDockerVM
 from gns3server.compute.docker.docker_error import DockerError, DockerHttp304Error, DockerHttp404Error, DockerHttp409Error
 
 log = logging.getLogger(__name__)
@@ -59,6 +60,16 @@ class Docker(BaseManager):
         self._session = None
         self._api_version = DOCKER_MINIMUM_API_VERSION
 
+    def _select_node_class(self, **kwargs):
+        """Select the node class based on console_type."""
+        if kwargs.get("console_type") == "docker_exec":
+            return VendorDockerVM
+        return DockerVM
+
+    async def create_node(self, name, project_id, node_id, *args, **kwargs):
+        self._NODE_CLASS = self._select_node_class(**kwargs)
+        return await super().create_node(name, project_id, node_id, *args, **kwargs)
+
     @staticmethod
     async def install_busybox(dst_dir):