This step will build an .apk using Gradle using this fantastic Docker image:
https://gitlab.com/fabmazz/android-fdroid-build-container/-/tree/ubuntu
Source code of this build step:
https://gitpull.it/source/libre-busto-ci-cd/browse/master/bin/fdroid-docker-builder.sh
TODO: Use the official image by Fabio Mazza, when the "DESIRED_UID" / "DESIRED_GID" features will be merged.