Add docker system prune command

This will hopefully keep the system drive cleaned up so we don't run issues with not enough disk space.

.github/workflows/publish_collab_image.yml

@@ -22,6 +22,9 @@ jobs:
       - name: Sign into DigitalOcean docker registry
         run: doctl registry login
 
+      - name: Prune Docker system
+        run: docker system prune
+
       - name: Checkout repo
         uses: actions/checkout@v3
         with: