fix(ci): prune Docker state before deploy to recover disk space

Previous builds leave cached layers and images on the runner. Add a
docker system prune at start of deploy to avoid container-creation
failures from disk exhaustion.

.gitea/workflows/deploy-gke.yaml

@@ -26,6 +26,13 @@ jobs:
       USE_GKE_GCLOUD_AUTH_PLUGIN: "True"
 
     steps:
+      - name: Free disk space
+        run: |
+          df -h /
+          docker system prune -af --volumes 2>/dev/null || true
+          rm -rf /tmp/.act-* /tmp/act-* 2>/dev/null || true
+          df -h /
+
       - name: Checkout
         uses: actions/checkout@v4