fix: resolve ENOSPC on CI by using BuildKit pip cache and registry layer cache

- Add `# syntax=docker/dockerfile:1` to enable BuildKit cache mounts
- Replace `--no-cache-dir` pip installs with `--mount=type=cache,target=/root/.cache/pip`
  so torch/CUDA wheels are reused across builds instead of re-downloaded every run
- Add `docker system prune -f` step before build to free dangling image/layer space
- Add `cache-from`/`cache-to` pointing to `:cache` tag so unchanged layers
  (including the heavy voice-deps layer) are pulled from registry instead of rebuilt

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -121,6 +121,9 @@ jobs:
           echo "value=$TAGS" >> $GITHUB_OUTPUT
           echo "build_version=$BUILD_VERSION" >> $GITHUB_OUTPUT
 
+      - name: Free disk space
+        run: docker system prune -f || true
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
 
@@ -138,3 +141,5 @@ jobs:
           push: true
           tags: ${{ steps.tags.outputs.value }}
           build-args: BUILD_VERSION=${{ steps.tags.outputs.build_version }}
+          cache-from: type=registry,ref=${{ env.IMAGE }}:cache
+          cache-to: type=registry,ref=${{ env.IMAGE }}:cache,mode=max