So I discover today that there is an option --ccache to request
flatpak-builder to compile using ccache, which is obviously a great idea
when rebuilding the same deps too often. Update the howto with the info.
build-export is actually a low-level tool used by flatpak-builder. When
using it directly, debug and locale extensions were not extracted as
separate extensions (unless tweaking complicated command lines), ending
up with a huge GIMP flatpak with the current procedure.
Since flatpak 0.9.5, the option --export-only has been added to
`flatpak-builder` so that the build and the export can be made in 2
separate steps while using the high level procedure.
See: https://github.com/flatpak/flatpak/issues/824
In particular, I didn't have the correct metadata because I was missing
appstream-compose. This should be a dependency of flatpak. But for the
time being, let's just have a note in our howto.
