Cloud applications typically integrate multiple components, each needing a virtualised runtime environment that provides the required software support (e.g., operating system, libraries). This paper shows how TOSCA and Docker can effectively support the orchestration of multi-component applications, even when their runtime specification is incomplete. More precisely, we first introduce a TOSCA-based representation of multi-component applications, and we illustrate how such representation can be exploited to specify only the application-specific components. We then present TosKeriser, a tool for automatically completing TOSCA application specifications, which can automatically discover the Docker-based runtime environments that provide the software support needed by the application components. We also show how we fruitfully exploited TosKeriser in two concrete case studies. Finally, we discuss how the specifications completed by TosKeriser can be automatically orchestrated by already existing TOSCA engines.