feat: keep Docker images on computes consistent with the controller host

Only relying on the image name lets a moved tag (e.g. a newer :latest)
silently serve stale content from a compute that already has an image
under the same name. When creating a Docker node, the controller now
pins the image id (Id from the Docker daemon on the controller host)
into the create payload. A compute holding a different image under the
same tag reports the image as missing, which routes it through the
image sync added by the previous commit and re-aligns the tag.

No new template fields or database changes: the controller host daemon
remains the source of truth and the pin is resolved per creation. When
the image is not available on the controller host the pin is omitted
and behavior is unchanged (the compute pulls from the repository).

gns3server/controller/node.py

@@ -465,6 +465,7 @@ class Node:
         data["node_id"] = self._id
         if self._node_type == "docker":
             timeout = None
+            await self._add_docker_image_digest(data)
         else:
             timeout = 1200
         trial = 0
@@ -788,6 +789,29 @@ class Node:
                 return True
         return False
 
+    async def _add_docker_image_digest(self, data):
+        """
+        Pin the Docker image of a node to the image id found on the Docker daemon
+        of the controller host, so that a compute holding a different image under
+        the same tag (e.g. a moved :latest) gets it re-synced instead of silently
+        reusing the stale copy. Not set when the image is not on the controller host.
+        """
+
+        from gns3server.compute.docker import Docker
+        from gns3server.compute.docker.docker_error import DockerError
+
+        image = data.get("image")
+        if not image:
+            return
+        try:
+            image_info = await Docker.instance().query("GET", f"images/{image}/json")
+        except DockerError:
+            # not available on the controller host: nothing to pin against
+            return
+        image_id = image_info.get("Id")
+        if image_id:
+            data["image_digest"] = image_id
+
     async def _sync_missing_docker_image(self, image):
         """
         Export a Docker image from the Docker daemon on the controller host and