docker: inject config files into containers via extra_configs

Add an `extra_configs` field (list of {target, content}) to the docker
node/template/appliance schemas. For each entry GNS3 writes `content` to a
file in the node working directory and bind-mounts it read-only at `target`
inside the container.

This lets a NOS appliance seed its startup config without rebuilding the
image: XRd points XR_FIRST_BOOT_CONFIG at an injected /firstboot.cfg, FRR at
/etc/frr/frr.conf, etc. The bind is a single-file mount applied at create
time, so it works for both the generic init.sh path and vendor nodes that
skip init.sh (console_type=docker_exec). Entries are only injected when
present, so ordinary nodes are unaffected.

The content can't go through `environment` (it is line-delimited, one var per
line), hence a dedicated field -- the same plumbing shape as extra_volumes.

gns3server/schemas/compute/docker_nodes.py

@@ -18,7 +18,7 @@ from pydantic import BaseModel, Field
 from typing import Optional, List
 from uuid import UUID
 
-from ..common import NodeStatus, CustomAdapter, ConsoleType, AuxType
+from ..common import NodeStatus, CustomAdapter, ConsoleType, AuxType, ExtraConfig
 
 
 class DockerBase(BaseModel):
@@ -43,6 +43,7 @@ class DockerBase(BaseModel):
     environment: Optional[str] = Field(None, description="Docker environment variables")
     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)")
     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")