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/schemas/compute/docker_nodes.py

@@ -26,7 +26,7 @@ class DockerBase(BaseModel):
     Common Docker node properties.
     """
 
-    @field_validator("start_command", "environment", "extra_hosts", mode="before")
+    @field_validator("start_command", "environment", "extra_hosts", "startup_config_content", mode="before")
     @classmethod
     def _empty_string_to_none(cls, value):
         # Web clients serialize empty form fields as "" while unset values are
@@ -59,6 +59,9 @@ class DockerBase(BaseModel):
     extra_hosts: Optional[str] = Field(None, description="Docker extra hosts (added to /etc/hosts)")
     extra_volumes: Optional[List[str]] = Field(None, description="Additional directories to make persistent")
     extra_configs: Optional[List[ExtraConfig]] = Field(None, description="Configuration files injected into the container (bind-mounted read-only)")
+    startup_config_content: Optional[str] = Field(
+        None, description="Startup-config content (IOL runner images: materialized into the node's NVRAM at start)"
+    )
     memory: Optional[int] = Field(None, ge=0, description="Maximum amount of memory the container can use in MB")
     cpus: Optional[float] = Field(None, ge=0, description="Maximum amount of CPU resources the container can use")
     custom_adapters: Optional[List[CustomAdapter]] = Field(None, description="Custom adapters")