fix: add disk cleanup to CI and reduce Docker layers

Runner runs out of disk space during discord-gateway build.
Add docker system prune before build and combine Dockerfile
layers to reduce disk usage.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -20,6 +20,9 @@ jobs:
           apt-get update -qq
           apt-get install -y -qq --no-install-recommends ca-certificates git docker.io
           git config --global http.sslVerify false
+          # Clean up runner disk before build
+          docker system prune -af --volumes 2>/dev/null || true
+          apt-get clean 2>/dev/null || true
 
       - name: Checkout
         run: |