docker: graceful stop for vendor NOS containers (SIGTERM + 60s grace)

DockerVM.stop() terminated containers with an immediate SIGKILL — fine
for init.sh-based containers whose state is persisted beforehand, but a
systemd NOS (Cisco XRd, SR Linux) needs a graceful shutdown and treats
the abrupt kill as an unclean shutdown (exit 137 on every stop).

Extract the final termination into _terminate_container() and override
it in VendorDockerVM: POST /containers/{id}/stop?t=60 sends SIGTERM and
waits for systemd to stop services; Docker itself SIGKILLs the
container once the grace period expires, so no fallback is needed.
Docker's 304 (already stopped) is swallowed.

gns3server/compute/docker/vendor_docker_vm.py

@@ -35,7 +35,7 @@ import shutil
 
 from gns3server.utils.asyncio.telnet_server import AsyncioTelnetServer
 from gns3server.compute.docker.docker_vm import DockerVM
-from gns3server.compute.docker.docker_error import DockerError, DockerHttp404Error
+from gns3server.compute.docker.docker_error import DockerError, DockerHttp304Error, DockerHttp404Error
 
 log = logging.getLogger(__name__)
 
@@ -138,6 +138,20 @@ class VendorDockerVM(DockerVM):
                 pass
             self._console_exec_writer = None
 
+    async def _terminate_container(self):
+        """
+        Override: vendor NOS containers run systemd and require a graceful
+        shutdown (e.g. Cisco XRd treats an abrupt SIGKILL as an unclean
+        shutdown). Send SIGTERM and wait up to 60 s for the services to stop;
+        Docker SIGKILLs the container itself once the grace period expires,
+        so no fallback kill is needed. The blocking stop call sits well
+        inside the manager's default 300 s query timeout.
+        """
+        try:
+            await self.manager.query("POST", f"containers/{self._cid}/stop", params={"t": 60})
+        except DockerHttp304Error:
+            pass  # already stopped
+
     async def start(self):
         await super().start()
         if self.status == "started" and not self._gns3_init: