Support local volumes

[lionello]

Add support for locally mapped volumes. This could be implemented by building a sidecar image from the mapped files.

version: '3'
services:
  nextjs:
    restart: always
    image: caddy:2.7.6
    command: ["caddy", "file-server", "--root", "/static", "--listen", ":80"]
    #x-defang-static-files: /static
    volumes:
      - ./:/static
    ports:
      - target: 80
        published: 8000
        mode: ingress

Currently this emits a warning.

[lionello]

See also #490

[lionello]

This requires volume support, before we're able to copy the local files into the created volume.