docker image tests failed due to dockerhub limitation

[wyli]

cron-docker-test couldn't pull from dockerhub:

  /usr/bin/docker pull projectmonai/monai:latest
  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
  Warning: Docker pull failed with exit code 1, back off 4.538 seconds before retry.
  /usr/bin/docker pull projectmonai/monai:latest
  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
  Warning: Docker pull failed with exit code 1, back off 3.538 seconds before retry.
  /usr/bin/docker pull projectmonai/monai:latest
  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
  Error: Docker pull failed with exit code 1

https://github.com/Project-MONAI/MONAI/runs/1567795300?check_suite_focus=true#step:2:14

cc @IsaacYangSLA @YuanTingHsieh @behxyz @yanchengnv

[wyli]

I've submitted an OSS project application to get dockerhub's support
https://www.docker.com/community/open-source/application

[danielschulz]

May I fix this issue?

[Nic-Ma]

May I fix this issue?

Of course, welcome to contribute if you know how to fix it.

Thanks in advance.