1. A method, comprising: prior to execution of a program in a virtual container, recording a set of files accessed during a phase change on a physical read-only storage medium in a manner predetermined to minimize the time needed to retrieve the set of files; and at the occurrence of the phase change, or prior to the occurrence of a phase change if the files are prefetched, accessing the physical read-only storage medium and retrieving the files, or allowing an operating system to access the physical read-only storage medium and retrieve the files as they are needed by an application.