Use Ubuntu 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