Enhance GPU build workflow with disk cleanup steps

Added pre-job and post-job steps for disk cleanup in the GPU build workflow.

.github/workflows/build_GPU.yml

@@ -23,6 +23,22 @@ jobs:
           version=$(grep -oP "subgen_version\s*=\s*'\K[^']+" subgen.py)
           echo "version=$version" >> $GITHUB_ENV
 
+      - name: Pre-job: free disk space
+        run: |
+          set -euo pipefail
+          echo "Disk before cleanup:"
+          df -h /
+          # Prune buildx builder data and docker caches (safe)
+          sudo docker builder prune --all --force || true
+          sudo docker system prune -af --volumes || true
+          # remove temp files
+          sudo rm -rf /tmp/* || true
+          # small safety: remove old Docker buildkit cache dirs (if present)
+          sudo rm -rf /var/lib/docker/buildkit || true
+          echo "Disk after cleanup:"
+          df -h /
+        shell: bash
+
       - name: Login to Docker Hub
         uses: docker/login-action@v3
         with:
@@ -37,3 +53,15 @@ jobs:
           tags: |
             mccloud/subgen:latest
             mccloud/subgen:${{ env.version }}
+
+      - name: Post-job: cleanup local docker artifacts
+        if: always()
+        run: |
+          set -euo pipefail
+          # remove images/containers/build cache used during this job to free disk
+          sudo docker builder prune --all --force || true
+          sudo docker image prune -af || true
+          sudo docker container prune -f || true
+          sudo docker system prune -af --volumes || true
+          df -h /
+        shell: bash