Launchpad PPAs do not support AppStream data in the sense that it is not globbed together for each package in a PPA and published to the repository in the way that it would be downloaded with an apt update.
To work around this and provide AppStream data for elementary packages available from the stable PPA, the build.sh script in this repository downloads a local mirror of the PPA and runs appstream-generator on it to generate the necessary metadata tarball. This is then packaged up as a .deb package and uploaded to the same PPA. When installed, it puts the metadata in a location where AppStream would expect to find it.
Building this metadata properly requires the elementary-icon-theme package to be installed so that the relevant icons can be extracted. So in a CI environment, an elementary Docker container is used. To build the metadata locally, you can use the following command:
docker run -i -v ${PWD}:/repo elementary/docker:stable /bin/bash -s bionic stable < build.sh
