Starting with 1 November 2020, Docker Hub limits the number of image pulls for anonymous users on a free plan. What this means for you is that when you use the Workflow Editor to set the image, no authentication is performed and your pull will be a pull by an anonymous user from the IP address of the Bitrise virtual machine.

Since the number of anonymous pulls is limited, you may not be able to start your build this way.

Use a Script Step! You can authenticate yourself with your own Docker Hub account through a Script Step, and then use the Step to pull the image you need.