Use Ubuntu Core cloud images as the baseline for built disk images. Overrides: * To use a non-default URL for downloading base Ubuntu cloud images, use the environment variable DIB\_CLOUD\_IMAGES * To download a non-default release of Ubuntu cloud images, use the environment variable DIB\_RELEASE * To use different mirrors rather than the default of archive.ubuntu.com and security.ubuntu.com, use the environment variable DIB\_DISTRIBUTION\_MIRROR