Add Datadog to library

First official container for the Datadog Agent.
Based on Agent 5.0.5.

I don't know what's the exact tagging policy, but because there is no reason to freeze the Datadog Agent to a specific major or minor version, the tag is there as an "indication" (that's why there is no "5" or "5.0" version).

Corresponding docs PR:
https://github.com/docker-library/docs/pull/104