Defang generate with Redis should automatically add x-defang-redis

[raphaeltm]

I generated a project with defang generate which ended up looking like this:

services:
  service1:
    restart: always
    build:
      context: .
      dockerfile: Dockerfile
    environment:
      - REDIS_HOST=redis
      - REDIS_PORT=6379

  redis:
    image: redis:alpine
    ports:
      - 6379:6379

Good starting point, but we should make sure that generated code includes x-defang-redis: true if it includes Redis at all.

[raphaeltm]

Also, even with x-defang-redis: true the code wouldn't work because of https://github.com/DefangLabs/defang/issues/547