use ExposedPorts as principal_port

Hi,

I found that some Docker images have only `ExposedPorts`, no `PortSpecs`; for example when I build a custom image using `Dockerfile` containing `EXPOSE ***`.

This patch is for these images. If there is no `PortSpecs` but `ExposedPorts`, the executer uses later it as a `principal_port`.

Thanks,
riywo