Add caching

Author: furtib

.github/workflows/buildbarn.yaml

@@ -40,6 +40,11 @@ jobs:
         with:
           version: latest
 
+      - name: Cache Docker images.
+        uses: ScribeMD/docker-cache@0.5.0
+        with:
+          key: docker-${{ runner.os }}-${{ hashFiles('.github/buildbarn/docker-compose.yml') }}
+
       - name: Pre-pull images
         working-directory: .github/buildbarn
         run: docker compose pull