[Docker log driver] cleanup output

[fearful-symmetry]

This PR addresses two issues: adding container labels and metadata to events, and making the output of the plugin ECS-compliant.

I added @henrikno to the review list, since I'd like input about any other bits of metadata that could be added.

Here's what the output looks like now:

{
  "_index": "dockerbeat-test",
  "_type": "_doc",
  "_id": "et0WgG4BjXmy5XAyo_2I",
  "_version": 1,
  "_score": null,
  "_source": {
    "@timestamp": "2019-11-18T19:56:36.635Z",
    "message": "This is a test log line",
    "container": {
      "labels": {},
      "id": "c7a30a2ab012c609dbb36fc8773ebcfb553827af4082c30d4f630c8ab79735dd",
      "name": "/fervent_black",
      "image": {
        "name": "debian:jessie"
      }
    }
  },
  "fields": {
    "@timestamp": [
      "2019-11-18T19:56:36.635Z"
    ]
  },
  "sort": [
    1574106996635
  ]
}