1. A system for efficiently building virtual appliances in a hosted environment, comprising: a build database configured to store a plurality of image archives, wherein the plurality of image archives each include a file system having one or more directories and a plurality of files installed within the one or more directories; and a build engine configured to: receive a build request that includes an image description for an appliance; create a file system layout from the image description, wherein the file system layout defines a directory structure for an operating system image; copy the file system for one of the plurality of image archives to the file system layout created from the image description, wherein the copied file system provides a subset of the operating system image; and build the operating system image, wherein the operating system image includes a file system having a plurality of files installed within one or more directories in accordance with the directory structure defined for the operating system image.