This has been resolved. The root cause was a Docker Hub incident: https://status.docker.com/pages/history/533c6539221ae15e3f000031. We have made changes that would mitigate the impact if a similar incident occurs in the future.

A fix has been implemented and we are monitoring the results.

Builds are failing with an error "unexpected status: 415 Unsupported Media Type".

It is believed to be affecting all Docker Builds in both US and EU. Engineers are aware of the issue and are working to resolve it.