1. A system for efficiently building virtual appliances in a hosted environment, comprising: a build database configured to store reusable image archives having file systems with one or more files installed in one or more directories, wherein the reusable image archives collect all the files installed in the one or more directories and preserve information relating to the one or more directories associated with the file systems; and a build engine configured to: receive a build request that includes an image description associated with a virtual appliance; create, from the image description associated with the virtual appliance, a root file system having a root directory structure that defines a layout associated with all files and directories to install in an operating system image associated with the virtual appliance; add all the files in the file system associated with one of the reusable image archives that provides a perfect subset of the root file system to the root directory structure in the operating system image associated with the virtual appliance; and build the operating system image associated with the virtual appliance, wherein the operating system image includes all the files and directories installed therein in accordance with the root directory structure that defines the layout associated therewith.