CI: Enable BuildKit cache mounts for faster npm installs

- Add syntax directive for BuildKit cache-mount support
- Cache /root/.npm between builds (web + server npm install)
- Enable DOCKER_BUILDKIT=1 in CI build step
- Remove aggressive builder prune that killed layer cache after each build

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

.forgejo/workflows/build-deploy.yml

@@ -47,7 +47,7 @@ jobs:
 
       - name: Build Docker image
         run: |
-          docker build \
+          DOCKER_BUILDKIT=1 docker build \
             --build-arg "VITE_BUILD_CHANNEL=${{ steps.vars.outputs.channel }}" \
             --build-arg "VITE_APP_VERSION=${{ steps.vars.outputs.version }}" \
             -t ${{ env.REGISTRY }}/${{ env.IMAGE }}:${{ steps.vars.outputs.tag }} \
@@ -75,7 +75,6 @@ jobs:
             docker rmi "${{ env.REGISTRY }}/${{ env.IMAGE }}:latest" 2>/dev/null || true
           fi
           docker image prune -f 2>/dev/null || true
-          docker builder prune -f --keep-storage=4GB 2>/dev/null || true
 
   deploy:
     runs-on: ubuntu-latest
@@ -125,7 +124,6 @@ jobs:
         if: always()
         run: |
           docker image prune -f 2>/dev/null || true
-          docker builder prune -f --keep-storage=4GB 2>/dev/null || true
 
   bump-version:
     runs-on: ubuntu-latest