Dockerhub is now reporting this issue as resolved on their status page:
https://status.docker.com/pages/533c6539221ae15e3f000031

Because of that status and our internal logging, we are moving this to resolved. If you continue to experience issues with Dockerhub, please reach out to CircleCI support.

Our internal logs are showing a reduced error rate from Dockerhub. We are continuing to monitor the situation.

Dockerhub is currently experiencing an incident. You can see more details about the incident here:
https://status.docker.com/pages/533c6539221ae15e3f000031

While core CircleCI services are unaffected, many customers make use of Docker images from Dockerhub. This incident could cause those jobs to fail. We are proactively creating this status page notification because of that and are actively monitoring the situation.