1. A method for the execution of immutable file system images on a computer system, the method comprising: establishing a root file system; forming a plurality of immutable file system images wherein each of the plurality of immutable file system images is associated with a mutually exclusive sub-tree of a file system tree; attaching each of a plurality of immutable file system images to an independent directory of the root file system prior to attaching other file systems to the root file system; generating a virtualization data structure corresponding to an empty file system tree; creating at run-time, in the virtualization data structure, a virtual directory including a virtual entry for each of the plurality of immutable file system images wherein each virtual entry includes file system structure of the mutually exclusive sub-tree; and joining functionally at run-time two or more of the plurality of immutable file system images by combining the virtual entries for each of the two or more of the plurality of immutable file system images.