feat: IOU-style startup-config for IOL Docker nodes

Templates reference a config file with the GNS3_IOL_STARTUP_CONFIG
environment knob; the controller materializes the file content into
startup_config_content on node creation (sent once, knob consumed —
the same pattern as the IOU startup_config mapping). The compute builds
the content into the node's nvram_<app id> at the next start using the
IOU nvram_import utility (IOL and IOU share the nvram container format,
verified against iol-xe 17.18.02): valid config at boot, no setup
dialog, %h hostname substitution, hostname rewrite on rename.

Semantics verified against the runner: IOL boots from NVRAM whenever it
holds a config, so a plain stop/start never re-applies the startup
config and 'write memory' survives restarts; an explicit content edit
(PUT) is re-applied on the next start and wins over the saved config,
like IOU.

gns3server/api/routes/compute/docker_nodes.py

@@ -140,6 +140,7 @@ async def update_docker_node(node_data: schemas.DockerUpdate, node: DockerVM = D
         "extra_hosts",
         "extra_volumes",
         "extra_configs",
+        "startup_config_content",
         "memory",
         "cpus",
     ]
@@ -147,7 +148,8 @@ async def update_docker_node(node_data: schemas.DockerUpdate, node: DockerVM = D
     changed = False
     node_data = jsonable_encoder(node_data, exclude_unset=True)
     for prop in props:
-        if prop in node_data and node_data[prop] != getattr(node, prop):
+        # hasattr: startup_config_content only exists on IOLDockerVM
+        if prop in node_data and hasattr(node, prop) and node_data[prop] != getattr(node, prop):
             setattr(node, prop, node_data[prop])
             changed = True
     # We don't call container.update for nothing because it will restart the container