ci: dockerhub login to increase rate-limit (#10727)

Docker Hub has rate-limits for anonymous users. This adds logging in,
which will increase the limit.

Example error message
```
Unable to find image 'mongo:6' locally
  docker: Error response from daemon: toomanyrequests: 
You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit.
  ```
  
 Example error in pipeline: https://github.com/payloadcms/payload/actions/runs/12911899766/job/36005722765?pr=10723

.github/workflows/main.yml

@@ -448,6 +448,14 @@ jobs:
           echo "POSTGRES_URL=postgresql://$POSTGRES_USER:$POSTGRES_PASSWORD@localhost:5432/$POSTGRES_DB" >> $GITHUB_ENV
         if: matrix.database == 'postgres'
 
+      # Login to Docker Hub in order to up rate-limit
+      - name: Login to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+        if: matrix.database == 'mongodb'
+
       - name: Start MongoDB
         uses: supercharge/mongodb-github-action@1.11.0
         with: