Building nightly flatpak apps in the CI

[Abderrahim Kitouni <akitouni gnome org>]

Dear GNOME developers,

we currently have a collection of apps that are continuously being 
built [1] and
published to sdk.gnome.org. We'd like to move this to the CI of 
individual apps so
that maintainers are aware of build failures as they occur, and to free 
up some
resources of our infrastructure.

Starting today, the repositories on sdk.gnome.org are considered 
deprecated, and
should disapear soon. (We'll be keeping it at least until 3.35.1 is 
released).
The new home for nightly flatpak builds of apps is 
https://nightly.gnome.org/repo/
The repository currently contains the runtimes and a single app, 
Epiphany.

For users, no configuration changes are needed. The gnome-nightly 
repository has
been redirected, the flatpak remote should be updated automatically. 
You only need
to reinstall apps from the gnome-nightly repository as they become 
available there.
You can find the flatpakref files here [2].

The transition should be very easy if you're already following the 
DevOps with
flatpak initiative [3] using the CI templates. The guide has been 
updated to
reflect the latest changes.

Please make sure your master branch is protected so you can use the 
secret token
defined on the GNOME group in gitlab. If your app isn't in the GNOME 
group and you
would like to publish the nightlies, please contact us on #release-team 
on IRC,
and we'll work something out.

If you would like to use different build flags, or a different 
application id
between the nightly build and the MR review builds, you can do so by 
using a
template for the common variables, and a different job between master 
and
non-master branches. See [4] for an example.

If you use a different branch name for your main developement branch
(e.g. mainline), you also need to rewrite the only clause in the 
nightly job. [5]

This builds only for x86_64 for now. i386 builds are going to be 
dropped, and ARM
builds will need to be added later once we have an ARM CI runner we can 
use for apps.

If you need help, please contact me or alatiera on IRC (you can find us 
on
#gnome-hackers or #release-team).

[1] https://gitlab.gnome.org/GNOME/gnome-apps-nightly
[2] https://nightly.gnome.org/repo/appstream/
[3] https://gitlab.gnome.org/GNOME/Initiatives/wikis/DevOps-with-Flatpak
[4] 
https://gitlab.gnome.org/GNOME/epiphany/commit/662b22c7560aa1fada8b853745d63fc377af7f80
[5] 
https://gitlab.gnome.org/GNOME/citemplates/blob/master/flatpak/flatpak_ci_initiative.yml#L132