Persist the old preview URLs for future jobs

[ChristianRimondi]

This uses the labels added in #4 to generate and persist a json file with a list of previews info associated with the containers removed during the current orb execution. The file format is something like:

{
  "PR_NUMBER": {
     "preview_url": "https://SUBDOMAIN.DOMAIN",
     "branch": "BRANCH"
  }
}

This adopts the same approach used in #3, so the way of retrieving the persisted file is the same:

  steps:
    - attach_workspace:
        # Must be absolute path or relative path from working_directory
        at: /tmp/workspace

    - run: |
        cat /tmp/workspace/deleted-previews.json

To generate the json file it uses docker inspect to retrieve the containers info in a json format and jq to parse this info:

# This selects only the containers with a label named `pull-request`
map(select(.Config.Labels["pull-request"]))

# This maps the container info returned by `docker inspect` into an array of
# `{"key": "PR_NUMBER", "value": {"branch": "BRANCH", "preview_url": "PREVIEW_URL"}}`
map({
  key: .Config.Labels["pull-request"],
  value: {
    branch: .Config.Labels["branch"],
    preview_url: "https://\(.Config.Labels["subdomain"]).<< parameters.domain >>"
  }
})

# Convert an array `[{"key": k, "value": v}]` into an object `{k: v}`
from_entries