Pull TiDB Docker images in parallel

Use background processes to pull tidb, pd, and tikv images simultaneously,
then wait for all pulls to complete. This reduces image pull time from
sequential (3x pull time) to parallel (max pull time).

After pulling, we still use Buildx to cache the images for future runs.

Author: zph

.github/workflows/main.yml

@@ -209,7 +209,23 @@ jobs:
         if: contains(matrix.target, 'tidb')
         uses: docker/setup-buildx-action@v3
 
-      - name: Pull and cache TiDB Docker images using Buildx
+      - name: Pull TiDB Docker images in parallel
+        if: contains(matrix.target, 'tidb')
+        run: |
+          VERSION="${{ steps.extract-tidb-version.outputs.version }}"
+          echo "Pulling TiDB component images for v${VERSION} in parallel..."
+          
+          # Pull all three images in parallel
+          docker pull pingcap/tidb:v${VERSION} &
+          docker pull pingcap/pd:v${VERSION} &
+          docker pull pingcap/tikv:v${VERSION} &
+          
+          # Wait for all pulls to complete
+          wait
+          
+          echo "All TiDB component images pulled successfully"
+      
+      - name: Cache TiDB Docker images using Buildx
         if: contains(matrix.target, 'tidb')
         uses: docker/build-push-action@v5
         continue-on-error: false
@@ -224,7 +240,7 @@ jobs:
           cache-from: type=gha,scope=tidb-v${{ steps.extract-tidb-version.outputs.version }}
           cache-to: type=gha,mode=max,scope=tidb-v${{ steps.extract-tidb-version.outputs.version }}
 
-      - name: Pull and cache PD Docker image using Buildx
+      - name: Cache PD Docker image using Buildx
         if: contains(matrix.target, 'tidb')
         uses: docker/build-push-action@v5
         continue-on-error: false
@@ -239,7 +255,7 @@ jobs:
           cache-from: type=gha,scope=pd-v${{ steps.extract-tidb-version.outputs.version }}
           cache-to: type=gha,mode=max,scope=pd-v${{ steps.extract-tidb-version.outputs.version }}
 
-      - name: Pull and cache TiKV Docker image using Buildx
+      - name: Cache TiKV Docker image using Buildx
         if: contains(matrix.target, 'tidb')
         uses: docker/build-push-action@v5
         continue-on-error: false