Dockerfile: use appstream generator from 3.16

The appstream generator is finally in good shape in stable and I compromise to keep it in that state.

Fixes alpine/infra/compose/appstream-generator#2 (closed) once deployed, since the bug has been fixed backporting the relevant commit to the appstream package that will be in 3.16.

EDIT: Dropping the caches is needed after the new deployment (removing the db directory in the persistent volume)

Draft until 3.16 is released :)