Docker support

This PR enables instrumentation on Docker (LXC/cgroups) containers for:
- Memory
- CPU
- Disk

It uses the native Unix socket to communicate with `dockerd`, and thus doesn't need any particular configuration.
It supports both old-style and new-style cgroups mountpoints, so it should work out of the box for any system that has cgroups support enabled.

To enable:
- make sure `dd-agent` is part of the `docker` group (so it can access the socket)
- rename `docker.yaml.example` to `docker.yaml`.