Propose to remove ToolDockerfile.descriptor

[delagoya]

Much like issue #21 having both a descriptor and url seems redundant and the url should be the canonical source.

Both of these seems like an optimization for a particular implementation of a the Dockstore UI.

[denis-yuen]

Like the suggestion in #21 I think it makes sense to make url optional rather than the canonical source.

[garyluu]

Looks like descriptor isn't present in ToolDockerfile:

  ToolDockerfile:
    type: object
    description: A tool dockerfile is a document that describes how to build a particular Docker image.
    required:
      - dockerfile
    properties:
      dockerfile:
        type: string
        description: The dockerfile content for this tool.
      url:
        type: string
description: 'Optional url to the dockerfile used to build this image, should include version information, and can include

Just the dockerfile contents and the optional url

[denis-yuen]

the proposed idea is that dockerfile is treated in the same way as descriptor in #21. Do not confuse that with the precise name of the property which is of course different.
Let's get that this in there
i.e.

make X.descriptor/dockerfile optional in the response if ToolDescriptor.url is provided.