PUT doesn't add ref to INPUT when registry mirror is set.

Disclaimer, this could be just because of a specific registry mirror, we use Artifactory.

Context: With the Docker Hub rate limit hell, we upgraded to Concourse 6.7.1 and setup a registry-mirror for docker-image and registry-image.

Issue: We have a build step that is then used by a task right after.

- name: build-pull-request
  plan:
  - get: pull-request
  - get: labels
  - put: ci-image
    inputs:
    - pull-request
    - labels
    get_params: { skip_download: true }
    params:
      cache: true
      build: pull-request
      tag_file: ./labels/tag_ci
      labels_file: ./labels/file.json
      target_name: dev
      build_args:
        ARTIFACTORY_CREDENTIALS: ((artifactory.username)):((artifactory.password))
- name: do-stuff
  plan:
    - get: ci-image
      passed: [build-pull-request]
      version: every
      trigger: true

And what happens is that when the put: ci-image is done, get: ci-image in do-stuff never gets its input.

We did the following:

- name: docker-image-no-mirror
  type: registry-image
  privileged: true
  source:
    repository: concourse/docker-image-resource
    tag: 1.5.1

And changed the type of ci-image to docker-image-no-mirror and VOILA, refs are filled correctly now.

Now, we still haven't looked at why, but this should help trace it at least.
Feel free to change the title, it's hard to explain that bug in a one liner :/

Start by reproducing the reported pipeline with a registry mirror and compare it with the docker-image-no-mirror configuration using concourse/docker-image-resource version 1.5.1. Trace how the put: ci-image result is passed to the following get: ci-image; done means the get receives its input and the behavior is consistent with and without the mirror.