docs: rebrand README + add Docker and bare-metal deploy

- Rewrite README with RetroBlog branding, config table, and two deploy
  paths (Docker Compose + bare-metal/systemd).
- Enable Next.js standalone output for slim runtime images.
- Add multi-stage Dockerfile (builds better-sqlite3 natively, runs as
  non-root, persists /app/data), docker-compose.yml, and .dockerignore.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,18 @@
+services:
+  retroblog:
+    build: .
+    image: retroblog:latest
+    container_name: retroblog
+    restart: unless-stopped
+    ports:
+      - "3000:3000"
+    environment:
+      # CHANGE THESE before exposing to the internet.
+      ADMIN_PASSWORD: ${ADMIN_PASSWORD:-admin}
+      ADMIN_SESSION_SECRET: ${ADMIN_SESSION_SECRET:-change-me-to-a-long-random-string}
+    volumes:
+      # Persist the SQLite database across container rebuilds.
+      - retroblog-data:/app/data
+
+volumes:
+  retroblog-data: