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/api/routes/compute/docker_nodes.py

@@ -78,6 +78,7 @@ async def create_docker_node(project_id: UUID, node_data: schemas.DockerCreate)
         aux_type=node_data.pop("aux_type", "none"),
         extra_hosts=node_data.get("extra_hosts"),
         extra_volumes=node_data.get("extra_volumes"),
+        extra_configs=node_data.get("extra_configs"),
         memory=node_data.get("memory", 0),
         cpus=node_data.get("cpus", 0),
     )
@@ -87,7 +88,8 @@ async def create_docker_node(project_id: UUID, node_data: schemas.DockerCreate)
     for key in (
         "console", "console_type", "console_resolution", "console_http_port",
         "console_http_path", "aux", "aux_type", "start_command", "environment",
-        "adapters", "mac_address", "extra_hosts", "extra_volumes", "memory", "cpus",
+        "adapters", "mac_address", "extra_hosts", "extra_volumes", "extra_configs",
+        "memory", "cpus",
     ):
         node_data.pop(key, None)
     for name, value in node_data.items():
@@ -137,6 +139,7 @@ async def update_docker_node(node_data: schemas.DockerUpdate, node: DockerVM = D
         "custom_adapters",
         "extra_hosts",
         "extra_volumes",
+        "extra_configs",
         "memory",
         "cpus",
     ]