Docker plugin

This plugin implements these components:

• docker.attachexec: Implements the Commander interface by sending commands through docker attach and reading the output.
• docker.logs: Implements the Streamer interface by reading the logs of the container. Writes are simply discarded.
• docker.stream: Implements the Streamer interface by attaching to the container.

All components share the same config:

host: "unix:///var/run/docker.sock"
container: container_name
timeout: 0s

Additionally, docker.logs supports an extra config stderr: true (default false) to read the stderr logs instead of the stdout logs.