I’ve resolved this but want to flag it up:
I was trying to upload a Dockerfile. As I already had a different Dockerfile in my local directory where I prepped course materials I called this new one Dockerfile-educative. With this I got build failures when I tried to upload with non-helpful error messages.
I suspected it might be objecting to me not calling the file simply Dockerfile. When I uploaded the same file but just called Dockerfile it then worked.