[gtk] (3 commits) Non-fast-forward update to branch ci-gtk3-update

[Emmanuele Bassi <ebassi src gnome org>]

The branch 'ci-gtk3-update' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  d0dfac4... ci: Add a Debian-based image
  a6705ec... ci: Add a Debian build
  81f50f7... ci: Build API reference separately

Commits added to the branch:

  43d1064... ci: Add a Debian-based image
  37587dd... ci: Add a Debian build
  f034977... ci: Build API reference separately