feat: add Docker support with self-hosted and hosted presets

Multi-stage Dockerfile (Node 22 Alpine), docker-compose for self-hosted
and hosted deployments, CI workflow for GHCR publishing, runtime env var
substitution, and cron sidecar with supercronic.

Aligns hosted extension preset with dev config (enable-banking,
ai-categorization, ai-chat, email).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,29 @@
+services:
+  app:
+    image: ghcr.io/gnubok/gnubok:latest
+    env_file: .env
+    ports:
+      - "${PORT:-3000}:3000"
+    security_opt:
+      - no-new-privileges:true
+    restart: unless-stopped
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:3000/api/health"]
+      interval: 30s
+      timeout: 5s
+      start_period: 10s
+      retries: 3
+
+  cron:
+    build:
+      context: docker
+      dockerfile: cron.Dockerfile
+    depends_on:
+      app:
+        condition: service_healthy
+    environment:
+      - CRON_SECRET=${CRON_SECRET}
+      - APP_URL=http://app:3000
+    volumes:
+      - ./docker/crontab.self-hosted:/etc/supercronic/crontab:ro
+    restart: unless-stopped