Add Docker and Nix packaging for the node

server/docker/: multi-stage image (rust:1-slim build, trixie runtime)
with all four binaries plus the pinned yggdrasil 0.5.14 .deb
(sha256-verified, fetched from the official release), a compose
example per household (host network, NET_ADMIN, /dev/net/tun,
init: true), a German-commented node.toml.example (safe default:
leech mode) and a README covering privileges, two-household wiring,
leech mode and the kill switch. Image build and smoke tests verified
locally with docker 29.8.1.

flake.nix: builds the workspace as one spidernet package via
rustPlatform.buildRustPackage (cargoLock against the committed
Cargo.lock) plus an experimental NixOS module; written without a
local Nix and marked accordingly.

server/.dockerignore

@@ -0,0 +1,9 @@
+# Keep the build context small; everything needed is Cargo.toml,
+# Cargo.lock and crates/ (see server/docker/Dockerfile).
+target/
+.git/
+docker/node.toml.example
+docker/node.toml
+docker/README.md
+docker/docker-compose.yml
+spidernet-state/