Adding linux capabilities doesn't work in Docker

Adding capabilities through `org.jclouds.docker.domain.Config.Builder.capAdd(...)` doesn't work. IMO it's because the configuration should be placed in HostConfig class instead.

Suggested fix
Move fields capAdd and capDrop from Config class to HostConfig class.

https://docs.docker.com/engine/reference/api/docker_remote_api_v1.22/#create-a-container