Update Volume syntax

[nicholasjackson]

We should add more flexibility to Container Volumes to add support for mounting volumes from other images, downloading archives and files, etc.

 # Other image 
  volume {
    source {
      image = "golang:latest"
      paths = ["/abc/go"]
    }
    
    destination = "/other"
  }

  # Internet archive (Gogetter)
  volume {
    source {
      url = "github.com/blah//something"
    }
    
    destination = "/other"
  }

  # Host files
  volume {
    source {
      paths = ["./files"]
    }
    
    destination = "/other"
  }