docker watcher proposal: use the events API instead of polling

**Replace polling by streaming the [events API](https://docs.docker.com/reference/api/docker_remote_api_v1.18/#monitor-dockers-events)**
- using the events API filter by the `start` event to get notified of all new containers that have started
- when a new container starts, check it against the list of criteria and register it if it matches
- removes the need to poll, and keep track of the list of previous containers
- the `/containers/json` endpoint that it uses now can be pretty slow with large numbers of containers

Start by tracing the watcher’s current polling of the /containers/json endpoint and how it applies criteria and registers matching containers. Use Docker’s events API with the start event, then verify that new matching containers are registered without polling or retaining a previous-container list.

Written by the indexing model from the issue text.