I have a auto build that is stuck, it was building for 2 hours and we tried to cancel it now its stuck cancelling.
I had the same problem today.
It will be marked as failed between 4-8 hours later.
After that you can build and try again.
Actually this problem pops up for me from time to time.
Note: If someone from the Docker team is reading this, it would be great if canceled jobs could be forced to shut down sooner. Especially since you need to wait for up to 8 hours until you can build again!
If you really need to release soon, build the image locally and push it by hand.
(Quick google) https://blog.shippable.com/build-a-docker-image-and-push-it-to-docker-hub