Ensure Buildx steps fail fast if TiDB images don't exist

Add continue-on-error: false to Buildx steps so they fail immediately
if Docker images don't exist, rather than silently continuing and
causing confusing errors later.

Author: zph

.github/workflows/main.yml

@@ -209,9 +209,10 @@ jobs:
         if: contains(matrix.target, 'tidb')
         uses: docker/setup-buildx-action@v3
 
-      - name: Cache TiDB Docker images using Buildx
+      - name: Pull and cache TiDB Docker images using Buildx
         if: contains(matrix.target, 'tidb')
         uses: docker/build-push-action@v5
+        continue-on-error: false
         with:
           context: .
           file: Dockerfile.tidb
@@ -223,9 +224,10 @@ 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: Cache PD Docker image using Buildx
+      - name: Pull and cache PD Docker image using Buildx
         if: contains(matrix.target, 'tidb')
         uses: docker/build-push-action@v5
+        continue-on-error: false
         with:
           context: .
           file: Dockerfile.tidb
@@ -237,9 +239,10 @@ 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: Cache TiKV Docker image using Buildx
+      - name: Pull and cache TiKV Docker image using Buildx
         if: contains(matrix.target, 'tidb')
         uses: docker/build-push-action@v5
+        continue-on-error: false
         with:
           context: .
           file: Dockerfile.tidb