Concourse extension doesn't recognize URL for "registry_mirror" with a docker-image resource

[guillaumBrisard]

URL for registry_mirror is not recognize URL for a docker-image resource

The extension wrongly reports for registry_mirror: http://docker-dev-virtual.repository.foo.intra

Expecting a 'Map' but found a 'Scalar'vscode-concourse(YamlSchemaProblem)

The documentation about registry_mirror for resource: https://github.com/concourse/docker-image-resource#source-configuration

[kdvolder]

I reproduced the problem with an example like this:

resources:
- name: testing
  type: docker-image
  source:
    registry_mirror: whatever

It looks like our Schema for that key expects it be something more like a object looking like this:

- name: testing
  type: docker-image
  source:
    repository: aaa
    registry_mirror:
      host: somehost.com
      password: aaa
      username: vvv

I'm not sure where we got that idea from, but clearly it does not agree with the documentation of the resource so it is a mistake.

Should be easy enough to fix.

[kdvolder]

I think we probably simply patterned the schema after the one from 'registry-image': https://github.com/concourse/registry-image-resource, mistakenly assuming it has the same schema.