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/api/routes/compute/docker_nodes.py

@@ -81,6 +81,7 @@ async def create_docker_node(project_id: UUID, node_data: schemas.DockerCreate)
         extra_configs=node_data.get("extra_configs"),
         memory=node_data.get("memory", 0),
         cpus=node_data.get("cpus", 0),
+        image_digest=node_data.get("image_digest"),
     )
     # Pop keys already consumed by create_node above so the setattr
     # fallback loop below only applies truly extra keys and does not
@@ -89,7 +90,7 @@ async def create_docker_node(project_id: UUID, node_data: schemas.DockerCreate)
         "console", "console_type", "console_resolution", "console_http_port",
         "console_http_path", "aux", "aux_type", "start_command", "environment",
         "adapters", "mac_address", "extra_hosts", "extra_volumes", "extra_configs",
-        "memory", "cpus",
+        "memory", "cpus", "image_digest",
     ):
         node_data.pop(key, None)
     for name, value in node_data.items():