A buildpackage configuration schema is as follows:
The buildpack to package. It must contain the following field:
A set of dependent buildpack locations, for packaging a meta-buildpack. Each dependent buildpack location must correspond to an order group within the meta-buildpack being packaged, and must have one of the following fields:
A URL or path to an archive, a packaged buildpack (saved as a
.cnb file), or a directory. If path is relative, it must be relative to the
A registry location (if no registry host is specified in the image name, DockerHub is assumed).
You can view sample buildpackages on Github.