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/docker_vm.py

@@ -1206,12 +1206,8 @@ class DockerVM(BaseNode):
 
             state = await self._get_container_state()
             if state != "stopped" and state != "exited":
-                # SIGKILL immediately. GNS3 has already persisted container state
-                # (permissions via _fix_permissions, /gns3volumes) before this
-                # point, and the business process (often an interactive shell)
-                # ignores SIGTERM — so a stop grace period buys nothing but latency.
                 try:
-                    await self.manager.query("POST", f"containers/{self._cid}/kill")
+                    await self._terminate_container()
                     log.debug(f"Docker container '{self._name}' [{self._image}] stopped")
                 except DockerHttp409Error:
                     # Container is already stopped
@@ -1222,6 +1218,18 @@ class DockerVM(BaseNode):
             return
         self.status = "stopped"
 
+    async def _terminate_container(self):
+        """
+        Final termination of a still-running container: immediate SIGKILL.
+        GNS3 has already persisted container state (permissions via
+        _fix_permissions, /gns3volumes) before this point, and the business
+        process (often an interactive shell) ignores SIGTERM — a stop grace
+        period buys nothing but latency. Vendor NOS containers override this
+        with a graceful SIGTERM shutdown (see VendorDockerVM).
+        """
+
+        await self.manager.query("POST", f"containers/{self._cid}/kill")
+
     async def pause(self):
         """
         Pauses this Docker container.