appliance: close the v8 gaps for Docker vendor appliances

DockerPropertiesV8 now accepts custom_adapters (already available to
v1-6 top-level appliances and to Qemu v8 properties), so port-named
Docker appliances (XRd, SR Linux) can move to the v8 format without
losing their interface naming.

Appliance.type resolves the node type from the v8 settings template_type
(default set first) instead of misclassifying every v8 appliance as
qemu, and _get_default_symbol applies the docker guest symbol to v8
Docker guest appliances.

gns3server/schemas/controller/appliances.py

@@ -481,6 +481,7 @@ class DockerPropertiesV8(BaseModel):
     extra_volumes: Optional[List[str]] = Field(
         None, title='Additional directories to make persistent'
     )
+    custom_adapters: Optional[List[CustomAdapterItem]] = Field(None, title='Custom adapters')
     extra_configs: Optional[List[ExtraConfig]] = Field(
         None, title='Configuration files injected into the container (bind-mounted read-only)'
     )