fix(ci): install docker buildx before PR image build

The self-hosted runner has no BuildKit/buildx installed. Add
docker/setup-buildx-action@v3 step to install it, then use
docker buildx build --load for the PR image build.

Author: fenio

.github/workflows/integration.yml

@@ -122,15 +122,18 @@ jobs:
         with:
           ref: refs/pull/${{ inputs.pr }}/merge
 
+      - name: Set up Docker Buildx
+        if: inputs.pr != ''
+        uses: docker/setup-buildx-action@v3
+
       - name: Build PR image
         if: inputs.pr != ''
-        env:
-          DOCKER_BUILDKIT: '1'
         run: |
           PR_TAG="pr-${{ inputs.pr }}"
           IMAGE="ghcr.io/fenio/tns-csi:${PR_TAG}"
           echo "=== Building image from PR #${{ inputs.pr }} ==="
-          docker build \
+          docker buildx build \
+            --load \
             --build-arg VERSION="0.0.0-${PR_TAG}" \
             --build-arg GIT_COMMIT="$(git rev-parse --short HEAD)" \
             --build-arg BUILD_DATE="$(date -u +%Y-%m-%dT%H:%M:%SZ)" \