feat: Docker image + ghcr publishing

- multi-stage Dockerfile (non-root, embeds SPA, default config, ~160MB) + .dockerignore
- .github/workflows/docker.yml: push to ghcr.io/infosave2007/cortiq-gateway
  (master -> :edge/:sha amd64; tags -> :X.Y.Z/:latest, linux/amd64+arm64)
- README (EN+RU): docker run instructions; CHANGELOG
- verified locally: image builds, container serves /healthz + /admin

README.md

@@ -51,6 +51,11 @@ One OpenAI-compatible endpoint → automatic model selection from your pool
 ## 📦 Install
 
 ```bash
+# Docker (GitHub Container Registry)
+docker run -p 9000:9000 -e CORTIQ_ADMIN_TOKEN=secret \
+  -v "$PWD/config:/app/config" ghcr.io/infosave2007/cortiq-gateway:latest
+# → admin console at http://localhost:9000/admin
+
 # from crates.io
 cargo install cortiq-gateway
 
@@ -60,6 +65,9 @@ cd cortiq-gateway
 cargo build --release   # ./target/release/cortiq-gateway
 ```
 
+The image ships a default config and runs out of the box; mount `/app/config` (with a
+`gateway.toml`) to use your own. Tags: `:latest`, `:X.Y.Z`, `:edge` (linux/amd64 + arm64).
+
 ---
 
 ## 🖥️ Admin console