ci: Add a CI check for REUSE-compliant licensing/copyright headers

This doesn’t enforce licensing/copyright headers to be present on all
files, but does check that at least a minimum number of files are
correct.

This should help avoid new files being added without appropriate
licensing information in future.

The baseline is set at what `reuse lint` outputs for me at the moment.

See https://reuse.software/tutorial/#step-2 for information about how to
add REUSE-compliant licensing/copyright to files.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1415

.gitlab-ci.yml

@@ -13,7 +13,7 @@ cache:
 variables:
   FEDORA_IMAGE: "registry.gitlab.gnome.org/gnome/glib/fedora:v19"
   COVERITY_IMAGE: "registry.gitlab.gnome.org/gnome/glib/coverity:v7"
-  DEBIAN_IMAGE: "registry.gitlab.gnome.org/gnome/glib/debian-stable:v13"
+  DEBIAN_IMAGE: "registry.gitlab.gnome.org/gnome/glib/debian-stable:v14"
   MINGW_IMAGE: "registry.gitlab.gnome.org/gnome/glib/mingw:v9"
   MESON_TEST_TIMEOUT_MULTIPLIER: 4
   G_MESSAGES_DEBUG: all
@@ -72,6 +72,17 @@ sh-and-py-check:
       - "**/*.py"
       - "**/*.sh"
 
+style-check-mandatory:
+  extends: .only-default
+  image: $DEBIAN_IMAGE
+  stage: style-check
+  allow_failure: false
+  script:
+    - .gitlab-ci/run-reuse.sh
+  variables:
+    # The submodules are needed for `reuse lint` to work properly
+    GIT_SUBMODULE_STRATEGY: "normal"
+
 fedora-x86_64:
   extends:
     - .build-linux