feat: add Docker support for palladium-lightning node

- Dockerfile.palladium-lightning: multi-stage build (ubuntu:22.04)
  builder stage compiles lightningd from source with git submodules;
  runtime stage copies palladium-cli from palladium-node:local image
  (no Rust plugins, no cross-compilation, amd64 only)

- docker-compose.lightning.yml: joins external `palladium-net`, exposes
  port 9735 (P2P), connects to palladiumd:2332 via RPC using credentials
  from .env

- .env.example: template for PALLADIUM_RPCUSER / PALLADIUM_RPCPASSWORD

- .gitignore: add .env to prevent accidental credential commits

docker-compose.lightning.yml

@@ -0,0 +1,46 @@
+# Palladium Lightning — docker-compose
+#
+# Joins the existing `palladium-net` network created by the palladiumd compose stack.
+# The palladium-node:local image must be built before this one (palladium-cli is copied from it).
+#
+# Usage:
+#   # 1. Create .env with RPC credentials (see .env.example)
+#   # 2. Ensure palladiumd is running on palladium-net
+#   docker compose -f docker-compose.lightning.yml build
+#   docker compose -f docker-compose.lightning.yml up -d
+#
+#   # Check status
+#   docker logs palladium-lightning
+#   docker exec palladium-lightning lightning-cli --network=palladium getinfo
+
+networks:
+  palladium-net:
+    external: true  # created by the palladiumd compose stack
+
+services:
+  palladium-lightning:
+    build:
+      context: .
+      dockerfile: Dockerfile.palladium-lightning
+    image: palladium-lightning:local
+    container_name: palladium-lightning
+    restart: unless-stopped
+    networks:
+      - palladium-net
+    ports:
+      - "0.0.0.0:9735:9735"   # Lightning P2P — must be reachable from the internet
+      # RPC (9835) is NOT exposed on the host.
+      # Use: docker exec palladium-lightning lightning-cli --network=palladium <command>
+    volumes:
+      - ./.lightning:/root/.lightning
+    environment:
+      LIGHTNINGD_NETWORK: palladium
+      EXPOSE_TCP: "false"
+    command: >
+      --network=palladium
+      --palladium-rpcconnect=palladiumd
+      --palladium-rpcport=2332
+      --palladium-rpcuser=${PALLADIUM_RPCUSER}
+      --palladium-rpcpassword=${PALLADIUM_RPCPASSWORD}
+      --palladium-cli=/usr/local/bin/palladium-cli
+      --log-level=info