Docker images itself
An additional option is to use Clair.
Clair is an open source project for the static analysis of vulnerabilities in application containers (currently including appc and docker).
In regular intervals, Clair ingests vulnerability metadata from a configured set of sources and stores it in the database. Clients use the Clair API to index their container images; this creates a list of features present in the image and stores them in thedatabase. Clients use the Clair API to query the database for vulnerabilities of a particular image; correlating vulnerabilities and features is done for each request, avoiding the need to rescan images. When updates to vulnerability metadata occur, a notification can be sent to alert systems that a change has occured.
Our goal is to enable a more transparent view of the security of container-based infrastructure. Thus, the project was named Clair after the French term which translates to clear, bright, transparent.