fix: add missing deploy/DOCKER.md for Docker workflow

The release workflow references ./deploy/DOCKER.md to update DockerHub
description, but the file was missing causing ENOENT error.

deploy/DOCKER.md

+# Sub2API Docker Image
+
+Sub2API is an AI API Gateway Platform for distributing and managing AI product subscription API quotas.
+
+## Quick Start
+
+```bash
+docker run -d \
+  --name sub2api \
+  -p 8080:8080 \
+  -e DATABASE_URL="postgres://user:pass@host:5432/sub2api" \
+  -e REDIS_URL="redis://host:6379" \
+  weishaw/sub2api:latest
+```
+
+## Docker Compose
+
+```yaml
+version: '3.8'
+
+services:
+  sub2api:
+    image: weishaw/sub2api:latest
+    ports:
+      - "8080:8080"
+    environment:
+      - DATABASE_URL=postgres://postgres:postgres@db:5432/sub2api?sslmode=disable
+      - REDIS_URL=redis://redis:6379
+    depends_on:
+      - db
+      - redis
+
+  db:
+    image: postgres:15-alpine
+    environment:
+      - POSTGRES_USER=postgres
+      - POSTGRES_PASSWORD=postgres
+      - POSTGRES_DB=sub2api
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+
+  redis:
+    image: redis:7-alpine
+    volumes:
+      - redis_data:/data
+
+volumes:
+  postgres_data:
+  redis_data:
+```
+
+## Environment Variables
+
+| Variable | Description | Required | Default |
+|----------|-------------|----------|---------|
+| `DATABASE_URL` | PostgreSQL connection string | Yes | - |
+| `REDIS_URL` | Redis connection string | Yes | - |
+| `PORT` | Server port | No | `8080` |
+| `GIN_MODE` | Gin framework mode (`debug`/`release`) | No | `release` |
+
+## Supported Architectures
+
+- `linux/amd64`
+- `linux/arm64`
+
+## Tags
+
+- `latest` - Latest stable release
+- `x.y.z` - Specific version
+- `x.y` - Latest patch of minor version
+- `x` - Latest minor of major version
+
+## Links
+
+- [GitHub Repository](https://github.com/weishaw/sub2api)
+- [Documentation](https://github.com/weishaw/sub2api#readme)