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/debian-stable.Dockerfile

@@ -68,6 +68,9 @@ ENV LANG=C.UTF-8 LANGUAGE=C.UTF-8 LC_ALL=C.UTF-8
 
 RUN pip3 install meson==0.60.3
 
+# FIXME: Once we use Debian Bookworm, we can just install the `reuse` package
+RUN pip3 install reuse==1.0.0
+
 ARG HOST_USER_ID=5555
 ENV HOST_USER_ID ${HOST_USER_ID}
 RUN useradd -u $HOST_USER_ID -ms /bin/bash user