[docker] support tagging for events

[robparrott]

Despite the documentation, the docker agent module does not include tags when creating events:

https://github.com/DataDog/dd-agent/blob/master/checks.d/docker.py#L321-L328

[LeoCavaille]

Hey @robparrott, thanks for opening this issue.
Can you:

• indicate what you're referring to by the documentation?
• tell us what kind of tags you'd like to see in docker events?

[ejholmes]

👍 for this.

Events pertaining to a container should be tagged with the same tags that metrics include. This would probably require keeping an internal mapping of container id -> tags for every container and pruning the list when containers die.

Having this feature would be great since it's currently not possible to filter docker events by image_name or container_name (or any docker labels if you're setting collect_labels_as_tags).