ci: check REUSE specification compliance

 .builds/alpine.yml    | 4 ++++
 .builds/archlinux.yml | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/.builds/alpine.yml b/.builds/alpine.yml
index 6b9bc45..9026c32 100644
--- a/.builds/alpine.yml
+++ b/.builds/alpine.yml
@@ -21,6 +21,7 @@ packages:
   - wayland-protocols
   - xwayland-dev
   - meson
+  - reuse
   - scdoc
   - tar
   - wget
@@ -55,3 +56,6 @@ tasks:
       zig fmt --check river/
       zig fmt --check build.zig
       zig fmt --check build.zig.zon
+  - reuse: |
+      cd river
+      reuse lint
diff --git a/.builds/archlinux.yml b/.builds/archlinux.yml
index 6bb448d..cd39403 100644
--- a/.builds/archlinux.yml
+++ b/.builds/archlinux.yml
@@ -18,6 +18,7 @@ packages:
   - xcb-util-renderutil
   - xcb-util-wm
   - xorg-xwayland
+  - reuse
   - seatd
   - tar
   - wget
@@ -52,3 +53,6 @@ tasks:
       zig fmt --check river/
       zig fmt --check build.zig
       zig fmt --check build.zig.zon
+  - reuse: |
+      cd river
+      reuse lint