feat: Add Docker support for easy distribution

- Multi-stage Dockerfile for minimal image size (~119MB)
- Includes PostgreSQL, MySQL, MariaDB client tools
- Non-root user (UID 1000) for security
- Docker Compose examples for all use cases
- Complete Docker documentation (DOCKER.md)
- Kubernetes CronJob examples
- Support for Docker secrets
- Multi-platform build support

Docker makes deployment trivial:
- No dependency installation needed
- Consistent environment
- Easy CI/CD integration
- Kubernetes-ready

docker-compose.yml

@@ -0,0 +1,88 @@
+version: '3.8'
+
+services:
+  # PostgreSQL backup example
+  postgres-backup:
+    build: .
+    image: dbbackup:latest
+    container_name: dbbackup-postgres
+    volumes:
+      - ./backups:/backups
+      - ./config/.dbbackup.conf:/home/dbbackup/.dbbackup.conf:ro
+    environment:
+      - PGHOST=postgres
+      - PGPORT=5432
+      - PGUSER=postgres
+      - PGPASSWORD=secret
+    command: backup single mydb
+    depends_on:
+      - postgres
+    networks:
+      - dbnet
+
+  # MySQL backup example
+  mysql-backup:
+    build: .
+    image: dbbackup:latest
+    container_name: dbbackup-mysql
+    volumes:
+      - ./backups:/backups
+    environment:
+      - MYSQL_HOST=mysql
+      - MYSQL_PORT=3306
+      - MYSQL_USER=root
+      - MYSQL_PWD=secret
+    command: backup single mydb --db-type mysql
+    depends_on:
+      - mysql
+    networks:
+      - dbnet
+
+  # Interactive mode example
+  dbbackup-interactive:
+    build: .
+    image: dbbackup:latest
+    container_name: dbbackup-tui
+    volumes:
+      - ./backups:/backups
+    environment:
+      - PGHOST=postgres
+      - PGUSER=postgres
+      - PGPASSWORD=secret
+    command: interactive
+    stdin_open: true
+    tty: true
+    networks:
+      - dbnet
+
+  # Test PostgreSQL database
+  postgres:
+    image: postgres:15-alpine
+    container_name: test-postgres
+    environment:
+      - POSTGRES_PASSWORD=secret
+      - POSTGRES_DB=mydb
+    volumes:
+      - postgres-data:/var/lib/postgresql/data
+    networks:
+      - dbnet
+
+  # Test MySQL database
+  mysql:
+    image: mysql:8.0
+    container_name: test-mysql
+    environment:
+      - MYSQL_ROOT_PASSWORD=secret
+      - MYSQL_DATABASE=mydb
+    volumes:
+      - mysql-data:/var/lib/mysql
+    networks:
+      - dbnet
+
+volumes:
+  postgres-data:
+  mysql-data:
+
+networks:
+  dbnet:
+    driver: bridge