cache-to failure shouldn't be fatal

[johnhoran]

Description

I'm using buildx with cache-to set to type=gha,mode=max. Occasionally the github cache has issues, and we failures where the image is built and pushed to its repository correctly, but when buildx tries to push to cache, it fails and therefore the entire build fails.

I'd argue this shouldn't be fatal, the image has still built correctly and has pushed to the upstream repository.

[tonistiigi]

Set ignore-error=true in your --cache-to options for this behavior.