Provide a CI bot that will post how to pull PR Docker Image

[tomkerkhove]

As part of #389 we will push all CI Docker images to tomkerkhove/promitor-agent-scraper-ci:PR{PR Number}.

This will allow reviewers, contributor and others to pull in the changed Docker image.

We should provide a CI bot that will post a message to every PR with instructions where the image is located and how to pull it.

This can be done by using Docker Hub webhooks & the GitHub REST API.

Everything in terms of this CI bot should end up in deploy/automation/ci.

[tomkerkhove]

First thought was to use a Logic App that gets triggered by the Docker Hub Webhook which gives following payload:

{
  "push_data": {
    "pushed_at": 1552391917,
    "images": [],
    "tag": "PR123",
    "pusher": "tomkerkhove"
  },
  "callback_url": "https://registry.hub.docker.com/u/tomkerkhove/promitor-scraper-ci/hook/2gcc3ie1e10gd44didedge4e150dc3h5j/",
  "repository": {
    "status": "Active",
    "description": "CI builds for Azure Monitor scraper for Prometheus",
    "is_trusted": false,
    "full_description": "",
    "repo_url": "https://hub.docker.com/r/tomkerkhove/promitor-scraper-ci",
    "owner": "tomkerkhove",
    "is_official": false,
    "is_private": false,
    "name": "promitor-scraper-ci",
    "namespace": "tomkerkhove",
    "star_count": 0,
    "comment_count": 0,
    "date_created": 1552383743,
    "repo_name": "tomkerkhove/promitor-scraper-ci"
  }
}

Based on push_data.tag we can identify the ID of the PR and post a new comment to it via GitHub REST API.