[gimp] build: replace `flatpak build-export` by `flatpak-builder --export-only`

[Jehan Pagès <jehanp src gnome org>]

commit 8e329d25fb9a2224cb9475ec84fe0d153956147b
Author: Jehan <jehan girinstud io>
Date:   Tue Jun 20 16:27:11 2017 +0200

    build: replace `flatpak build-export` by `flatpak-builder --export-only`
    
    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

 build/flatpak/flatpak-howto.txt |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)
---
diff --git a/build/flatpak/flatpak-howto.txt b/build/flatpak/flatpak-howto.txt
index 9e51b40..cd92a29 100644
--- a/build/flatpak/flatpak-howto.txt
+++ b/build/flatpak/flatpak-howto.txt
@@ -3,6 +3,7 @@ GIMP Flatpak HowTo
 
 * Install flatpak and flatpak-builder. There are packages for most
   distributions. Check: http://flatpak.org/getting.html
+  Use at least flatpak 0.9.5.
 
 * Make sure `appstream-compose` is installed as well. This is used to parse the
   appdata file and generate the appstream (metadata like comments, etc.).
@@ -91,10 +92,13 @@ Or if you already have a build:
 
 * Export the 2 builds:
 
-> flatpak build-export --gpg-sign=YOUR_GPG_KEY /path/to/repo gimp-flatpak-build-x86-64
-> flatpak build-export --gpg-sign=YOUR_GPG_KEY /path/to/repo gimp-flatpak-build-i386
+> flatpak-builder --export-only --gpg-sign=YOUR_GPG_KEY --repo=/path/to/repo --arch=x86_64 
gimp-flatpak-build-x86-64 org.gimp.GIMP.json
+> flatpak-builder --export-only --gpg-sign=YOUR_GPG_KEY --repo=/path/to/repo --arch=i386 
gimp-flatpak-build-i386 org.gimp.GIMP.json
 
+  If you key was protected with a passphrase, you will need to input it.
   The same repository will contain all the builds.
+  Note: do not use `flatpak build-export` which is an internal low-level
+  command. See https://github.com/flatpak/flatpak/issues/824
 
 * Test it locally with: