Add ExtraHosts to HostConfig

Introduced by this PR: https://github.com/docker/docker/commit/68e48b65a64df10fc797cbaa89d6caa2188eadc9

You can see it in Docker's `HostConfig` struct here: https://github.com/docker/docker/blob/f98a1f1f7d9b3ef10c13fc3b6438c978b4d6aa78/runconfig/hostconfig.go#L52