ci: retry Docker Hub login to tolerate transient docker.io timeouts

One matrix job intermittently failed at the login step with
'Get "https://registry-1.docker.io/v2/": context deadline exceeded'
while sibling jobs authenticated fine with the same secrets — a
transient network blip, not a credentials problem. Replace
docker/login-action with a `docker login --password-stdin` retry
loop (5 attempts, 10s backoff) so transient failures self-heal.

.github/workflows/docker-build.yml

@@ -51,10 +51,24 @@ jobs:
         run: echo "TAG=$(date +'%Y%m%d.%H%M')-${GITHUB_SHA::7}" >> $GITHUB_OUTPUT
 
       - name: Login to Docker Hub
-        uses: docker/login-action@v3
-        with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_TOKEN }}
+        env:
+          DOCKERHUB_USERNAME: ${{ secrets.DOCKERHUB_USERNAME }}
+          DOCKERHUB_TOKEN: ${{ secrets.DOCKERHUB_TOKEN }}
+        run: |
+          # Retry to tolerate transient docker.io timeouts, e.g.
+          # 'Get "https://registry-1.docker.io/v2/": context deadline exceeded'.
+          # Credentials are written to ~/.docker/config.json, which buildx
+          # (docker/build-push-action) reads for the push.
+          for attempt in 1 2 3 4 5; do
+            if echo "$DOCKERHUB_TOKEN" | docker login -u "$DOCKERHUB_USERNAME" --password-stdin; then
+              echo "Logged in to Docker Hub on attempt $attempt"
+              exit 0
+            fi
+            echo "Docker Hub login attempt $attempt failed; retrying in 10s..."
+            sleep 10
+          done
+          echo "::error::Docker Hub login failed after 5 attempts"
+          exit 1
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3